Python 3 / JavaScript (V8), 87 bytes

p="'";f=';print("p="+f[7]+p+f[7]+";f="+p+f+p+f)';print("p="+f[7]+p+f[7]+";f="+p+f+p+f)

Try it online - Python!

Try it online - Javascript!

Vyxal D, 70 bytes/ Python 3, 77 bytes

a="`q‛:Ė+34Cp‛a=print('a=%r;exec(a[13:])#\\140:Ė'%a)";exec(a[13:])#`:Ė

Vyxal : Try it Online!

a="`q‛:Ė+34Cp‛a=print('a=%r;exec(a[13:])#\\140:Ė'%a)";exec(a[13:])#`:Ė

Python : Try it online!

The difference beetween the number of bytes is due to the fact that vyxal and python doesn't use the same charset

Explanation :

• Vyxal

a="                            # do nothing revelent here
   `q‛:Ė+                 `:Ė  # structure for a quine
         34Cp‛a=p              # prepend `a="` to the string 
                 rint          # do stuff that somehow push 0 to the stack
                     (         # for loop over 0 (do nothing)
                        #      # comment and implicily close the loop
                               # implicit output

• Python

The quine is a variation of a="print('a=%r;exec(a)'%a)";exec(a)

Wumpus/><>/Befunge-98 31 bytes

"]#34[~#28&o@,k+deg0 #o#!g00

Try it in Wumpus!, Try it in ><>!, Try it in Befunge-98!

How it Works:

Wumpus Code:

  " Start string literal
    Bounce off end of line and come back
  " End string literal
    ]      Push top of stack to bottom
     #34   Push double quote
        [~ Get bottom of stack and swap it with the double quote
          #31    Push 31
             &o@ Print the top 31 items on stack and terminate program

><> Code:

  "    Start string literal
       Wrap when it reaches the end of the line
  "    End string literal
    ]# Clear stack and reflect
  "    Wrapping string literal again, but backwards
                        +2: Copy the space and add 2 to get "
                    #o#!    Skip into the printing loop
                            Exit with an error

Befunge-98 Code:

  "   Wrapping string literal
    ] Turn right
    ] Turn right again, going West
  "   Wrapping string literal going West
                    !+2: Get double quote and invert it
                 #o#     Skip over the o instruction
            +2:$         Get double quote
         +ff             Push 30
      @,k                Print 30+1 items from the stack and terminate program.

Perl 5/Ruby/JavaScript (Node.js)/Bash/Python 2/PHP, 1031 bytes

s=1//2;_=r'''<?#/.__id__;s=+0;#';read -d '' q<<'';s=\';Q='echo s=1//2\;_=r$s$s$s\<\?\#/.__id__\;s=+0\;#$s\;read -d $s$s q\<\<$s$s\;s=\\$s\;Q=$s$Q$s\;eval\ \$Q;echo $q';eval $Q
$_='eval("0"?0?"def strtr(s,f,t);s.tr(f,t) end;class String;def chr(n);self+n.chr end;end":"$u=strtr=(s,f,t)=>[...f].reduce((s,a,i)=>s.replace(RegExp(a,`g`),t[i]),s);printf=console.log;(S=String).prototype.chr=function(n){return this+S.fromCharCode(n)}":[]&&"sub strtr{eval q(q(X)=~y/X/X/r)=~s/X/shift/ger}");printf(strtr("%s<?#/.__id__;s=+0;#j;read -d jj q<<jj;s=zj;Q=jecho s=1//2z;_=rksksksz<z?z#/.__id__z;s=+0z;#ksz;read -d ksks qz<z<ksksz;s=zzksz;Q=kskQksz;evalz zkQ;echo kqj;eval kQwk_=j%sj;eval(k_);//;#jjj;f=jjjs=1//2;_=r%%s%%s%%s;f=%%s%%s%%s;q=_[18]*3;print f%%%%(q,_,q,q,f,q)jjj;q=_[18]*3;print f%%(q,_,q,q,f,q)%s","jkwz","".chr(39).chr(36).chr(10).chr(92).chr(92)),[]&&"s=1//2;_=r".chr(39).chr(39).chr(39),$_,$u?"":"".chr(10));';eval($_);//;#''';f='''s=1//2;_=r%s%s%s;f=%s%s%s;q=_[18]*3;print f%%(q,_,q,q,f,q)''';q=_[18]*3;print f%(q,_,q,q,f,q)

Verify it online!

Based on my updates to this answer, I thought I'd try and optimise out the code that prints a different permutation, but ended up adding in Bash, which added a load more bytes anyway. Whilst this is more optimised than my first attempt (saved over 300 bytes) I'm sure it can still be golfed further.

Alternative Perl 5/Ruby/JavaScript (Node.js)/Bash/Python 2/PHP, 1040 bytes

s=1//2;_=r'''<?#/.__id__;s=+0;#';read -d '' q<<'';s=\';Q='echo s=1//2\;_=r$s$s$s\<\?\#/.__id__\;s=+0\;#$s\;read -d $s$s q\<\<$s$s\;s=\\$s\;Q=$s$Q$s\;eval\ \$Q;echo $q';eval $Q
$_='$z=0?"$&".next+92 .chr+10 .chr: 0..a||eval("printf=console.log;unescape`$%27%5C%0Ak`");$q=$z[1]?$z[1]:h^O;printf("%s%s%s%s<?#/.__id__;s=+0;#%s;read -d %s%s q<<%s%s;s=%s%s;Q=%secho s=1//2%s;_=r%ss%ss%ss%s<%s?%s#/.__id__%s;s=+0%s;#%ss%s;read -d %ss%ss q%s<%s<%ss%ss%s;s=%s%s%ss%s;Q=%ss%sQ%ss%s;eval%s %s%sQ;echo %sq%s;eval %sQ%s%s_=%s%s%s;eval(%s_);//;#%s%s%s;f=%s%s%ss=1//2;_=r%%s%%s%%s;f=%%s%%s%%s;q=_[18]*3;print f%%%%(q,_,q,q,f,q)%s%s%s;q=_[18]*3;print f%%(q,_,q,q,f,q)%s",[]&&"s=1//2;_=r",$r=[]&&$q,$r,$r,$q,$q,$q,$q,$q,$b=$z[2]?$z[2]:chr(92),$q,$q,$b,$d=$z[0]?$z[0]:h^L,$d,$d,$b,$b,$b,$b,$b,$d,$b,$d,$d,$b,$b,$d,$d,$b,$b,$b,$d,$b,$d,$d,$d,$b,$b,$b,$d,$d,$q,$d,$n=$z[3]?$z[3]:chr(10),$d,$q,$_,$q,$d,$q,$q,$q,$q,$q,$q,$q,$q,$q,$z[4]?"":$n);';eval($_);//;#''';f='''s=1//2;_=r%s%s%s;f=%s%s%s;q=_[18]*3;print f%%(q,_,q,q,f,q)''';q=_[18]*3;print f%(q,_,q,q,f,q)

Verify it online!

A little closer to my original approach, but the repetition of the args for printf is still insane. Using positional arguments instead make this only work in Chrome and is tricky to get working in PHP as well because the $s in %1$s is interpolated, but could save a lot of bytes, perhaps using a combination of the two approaches...

Perl 5/Ruby/PHP/JavaScript (Browser), 153 bytes

$_='$z=0?"$&".next: 0..a||eval("printf=console.log;atob`JCc`");printf("%s_=%s%s%s;eval(%s_);",$d=$z[0]?$z[0]:h^L,$q=$z[1]?$z[1]:h^O,$_,$q,$d);';eval($_);

Try the Perl online!
Try the Ruby online!
Try the PHP online!

$_='$z=0?"$&".next: 0..a||eval("printf=console.log;atob`JCc`");printf("%s_=%s%s%s;eval(%s_);",$d=$z[0]?$z[0]:h^L,$q=$z[1]?$z[1]:h^O,$_,$q,$d);';eval($_);

Perl/Javascript (SpiderMonkey), 106 bytes

$_='$q=+[]?h^O:unescape("%27");print("$_="+$q+$_+$q+";eval($_)"||(q($_),"=$q$_$q;",q(eval($_))))';eval($_)

Try the Perl online!
Try the JavaScript online!

Explanation

The quine data is stored in $_ in both languages and then evaled, which is pretty much standard procedure in Perl. I chose SpiderMonkey on TIO as it has a print function, but this could easily be ported to browser for + 20 bytes (add eval("print=alert"); to the beginning of $_s definition).

Perl sees the data stored in $_ and evals it as usual. Since +[] is truthy in Perl,' is stored in $q via the stringwise-XOR of h and O. The final trick is in the call to print where the first part for JavaScript uses +, which in Perl treats all items as numbers, and adds up to 0, then we use the || operator to return what we actually want (q($_),"=$q$_$q;",q(eval($_))) which is equivalent to "\$_=$q$_$q;eval(\$_)".

In JavaScript, +[] returns 0, so we call unescape("%27") to store ' in $q (unfortunately, atob doesm't exist in SpirderMonkey...). In the call to print, since + is the concatenation operator in JavaScript, the first block builds the desired output and the second part after the || is ignored.

Thanks to Patrick Roberts' comment for the unescape trick!

Perl/JavaScript (Browser), 108 bytes

$_='eval("q=_=>_+``;printf=console.log");printf(q`$_=%s%s%s;eval($_)`,$q=+[]?h^O:atob("Jw"),$_,$q)';eval($_)

Try the Perl online!

$_='eval("q=_=>_+``;printf=console.log");printf(q`$_=%s%s%s;eval($_)`,$q=+[]?h^O:atob("Jw"),$_,$q)';eval($_)

Explanation

We store the quine data in $_ in both languages and then eval it, which is pretty much standard procedure in Perl.

Perl sees the data stored in $_ and evals it as usual. The eval within $_ is executed and fails to parse, but since it's eval, doesn't error. printf is then called, with a single quoted string q(), with ` as the delimter, as just using ` would result in commands being executed in shell, then for the first use of $q, since +[] is truthy in Perl, ' is stored in $q via stringwise-XOR of h and O.

In JavaScript, the eval block within $_ sets up a function q, that returns its argument as a String and aliases console.log to printf, since console.log formats string like printf in Perl. When printf is called +[] returns 0, so we call atob to decode ' and store in $q.

C#/Java, 746 bytes

I use the property that chars in Java can be written as identical unicode sequences. If we have A instruction for C# compiler and B instruction for Java, we can use the following code fragment:

//\u000A\u002F\u002A
A//\u002A\u002FB

It will be "recognized" by the following way with C#:

//\u000A\u002F\u002A
A//\u002A\u002FB

And by the following way by Java:

//
/*
A//*/B

Because of \u000A is line break, \u002F is / and \u002A is * in Java.

So the final polyglot-quine is:

//\u000A\u002F\u002A
using System;//\u002A\u002F
class Program{public static void//\u000A\u002F\u002A
Main//\u002A\u002Fmain
(String[]z){String s="//@#'^using System;//'#^class Program{public static void//@#'^Main//'#main^(String[]z){String s=!$!,t=s;int[]a=new int[]{33,94,38,64,35,39,36};String[]b=new String[]{!&!!,!&n!,!&&!,!&@!,!&#!,!&'!,s};for(int i=0;i<7;i++)t=t.//@#'^Replace//'#replace^(!!+(char)a[i],b[i]);//@#'^Console.Write//'#System.out.printf^(t);}}",t=s;int[]a=new int[]{33,94,38,64,35,39,36};String[]b=new String[]{"\"","\n","\\","\\u000A","\\u002F","\\u002A",s};for(int i=0;i<7;i++)t=t.//\u000A\u002F\u002A
Replace//\u002A\u002Freplace
(""+(char)a[i],b[i]);//\u000A\u002F\u002A
Console.Write//\u002A\u002FSystem.out.printf
(t);}}

However, the size is too huge because of languages verbosity.

Compilation available on ideone.com: C#, Java.

C/Lisp, 555 bytes

t(setq /*;*/){}main(){char q='\"',s='\\';char*a= 
"~%t(setq /*;*/){}main(){char q='~A';char*a= 
~S;char*b=/* 
)(setq a ~S) 
(setq */ ~S;printf(b,s,q,s,s,q,a,q,q,s,s,s,q,s,s,s,s,q,q,b,q/* 
)(format t /* a /* a */);}~%";char*b=/* 
)(setq a "\\\"',s='\\\\") 
(setq */ " 
t(setq /*;*/){}main(){char q='%c%c',s='%c%c';char*a= 
%c%s%c;char*b=/* 
)(setq a %c%c%c%c%c',s='%c%c%c%c%c) 
(setq */ %c%s%c;printf(b,s,q,s,s,q,a,q,q,s,s,s,q,s,s,s,s,q,q,b,q/* 
)(format t /* a /* a */);} 
";printf(b,s,q,s,s,q,a,q,q,s,s,s,q,s,s,s,s,q,q,b,q/* 
)(format t /* a /* a */);} 

Intentionally blank first line.

C/dc, 152 bytes

z1d//[[z1d//]P91PP93P[dx]Pq 
;main(){char*a="z1d//[[z1d//]P91PP93P[dx]Pq%c;main(){char*a=%c%s%c;printf(a,10,34,a,34);}//]dx";printf(a,10,34,a,34);}//]dx

Taking advantage of comments, yeah!

C/PHP, 266 304 300 282 241 203 + 10 bytes

//<?php
function main($a){printf($a="%c//<?php%cfunction main(%ca){printf(%ca=%c%s%c,13,10,36,36,34,%ca,34,36,10,10,10);}%c#if 0%cmain();%c#endif",13,10,36,36,34,$a,34,36,10,10,10);}
#if 0
main();
#endif

+10 bytes because compiling in C requires the GCC compiler flag -Dfunction=.

How it works (in PHP):

• The PHP interpreter simply prints everything before the <?php as HTML. // is not a comment in HTML, so it's simply printed.
• main is declared as a function with a variable a.
• printf prints a carriage return (to override the already-printed //) and then the source code, using a standard C/PHP quining method.
• #if 0 is ignored by PHP.
• main($a) initializes an empty variable a. (Previously used error_reporting(0) to ignore errors caused by calling main())
• #endif is also ignored by PHP.

How it works (in C):

• //<?php is a single-line comment, so it is ignored.
• The function keyword is ignored due to the command-line compiler argument -Dfunction=.
• GCC and Clang don't care if variables start with or contain $. (This saved the day.)
• printf prints a carriage return (useless in this instance) and then the source code, using a standard C/PHP quining method.
• #if 0 ignores everything until the endif, so PHP can call main.
• #endif ends the "ignore me" block.

><> and CJam, 165 bytes

"~~~~~~~~~~~~~~~~~~~~~~~r00gol?!v93*0.Hi
'                               <       .1*5av!?log10oar~~~r
'"`{"`"\"_~e#.21 <.2+4*96;!?log10oa"}_~e#.21 <.2+4*96;!?log10oa

To CJam, the program starts with a multi-line string literal. This is escaped with `, and then it uses the standard quine to print the quine code, as well as a trailing comment.

To ><>, the first " starts a string literal that goes through the entire first row, pushing every character to the stack. After that, the trailing spaces (created due to the input being padded) are deleted, and then the stack is reversed. Every character in the stack (i.e. the entire first row) is output, and then it moves down to the second row.

The second row essentially does the same thing, except that it's in the opposite direction, so you don't need to reverse the stack. (I do anyway, because I have to delete the trailing spaces.)

Finally, it moves on to the third line. The only major difference is that you must skip the CJam block, which is done using . The single quote captures the entire line (again, backwards), and then it is output.

C/Vim 4.0, 1636 bytes

Contains control characters.

map () {}/*
map g ;data0df"f"cf"
f"cf"
f"D2kyyP;g6k2dd4x5jA"JxA","JxA","jyyPkJxA"jok;g2kdd4xkJx3jdd
map ;g O"vdldd0i# 0# 1# 2# 3# 4# 5# #0lx2lx2lx2lx2lx2lx2lx:s/##/#/g
o:s//"/gk0y2lj02lp"addk@ao:s//\\/gk0ly2lj02lp"addk@ao:s///gk04ly2lj02lp05l"vp"addk@ao:s///gk05ly2lj02lp05l"vp"vp"addk@ao:s//
/gk06ly2lj02lp05l"vp"vp"vp"addk@ao:s//
/gk02ly2lj02lp05l"vp"addk@a
unmap ()

map ;data o*/ char*g[]={"map () {}/*#2map g ;data0df#0f#0cf#0#5#3f#0cf#0#5#3f#0D2kyyP;g6k2dd4x5jA#0#3JxA#0,#0#3JxA#0,#0#3jyyPkJxA#0#3jo#3k;g2kdd4xkJx3jdd#2map ;g O#4#4#4#4#3#0vdldd0i## 0## 1## 2## 3## 4## 5## ###30lx2lx2lx2lx2lx2lx2lx:s/####/##/g#5o:s//#0/g#3k0y2lj02lp#0addk@ao:s//#1#1/g#3k0ly2lj02lp#0addk@ao:s//#4#4#3/g#3k04ly2lj02lp05l#0vp#0addk@ao:s///g#3k05ly2lj02lp05l#0vp#0vp#0addk@ao:s//#4#4#5/g#3k06ly2lj02lp05l#0vp#0vp#0vp#0addk@ao:s//#4#4#5/g#3k02ly2lj02lp05l#0vp#0addk@a#2unmap ()#2#2map ;data o*/ char*g[]={","#A#0#a#0,#0#b#0,#0#c#0#C#2","}; /*#3#2#2#0*/  print(char*s){char*t=s,c,d;while(c=*t++)if(c==35){c=*t++;if(c==35)putchar(c);else if(c==48)putchar(34);else if(c==49)putchar(92);else if(c==50)printf(#0#1n#0);else if(c==51)putchar(27);else if(c==52)putchar(22);else if(c==53)putchar(13);else if(c>64&&c<91)print(g[c-65]);else printf(g[c-97]);}else putchar(c);}  main(){print(g[1]);}"}; /*

"*/  print(char*s){char*t=s,c,d;while(c=*t++)if(c==35){c=*t++;if(c==35)putchar(c);else if(c==48)putchar(34);else if(c==49)putchar(92);else if(c==50)printf("\n");else if(c==51)putchar(27);else if(c==52)putchar(22);else if(c==53)putchar(13);else if(c>64&&c<91)print(g[c-65]);else printf(g[c-97]);}else putchar(c);}  main(){print(g[1]);}

Your Vim needs to have the following set:

set noai
set wm=0
set nosi
set tw=0
set nogdefault

C/TCL, 337 bytes

#define set char*f= 
#define F 
#define proc main(){ 
set F "#define set char*f= 
#define F 
#define proc main(){ 
set F %c%s%c; 
proc /* {} {} 
puts -nonewline %cformat %cF 34 %cF 34 91 36 36] 
set a {*/printf(f,34,f,34,91,36,36);} 
"; 
proc /* {} {} 
puts -nonewline [format $F 34 $F 34 91 36 36] 
set a {*/printf(f,34,f,34,91,36,36);} 

reticular/befunge-98, 28 bytes [noncompeting]

<@,+1!',k- ';';Oc'43'q@$;!0"

Try reticular! Try befunge 98!

Anything in between ;s in befunge is ignored, and ! skips into the segment between ;s for reticular. Thus, reticular sees:

<@,+1!',k- ';';Oc'43'q@$;!0"
<                             move left
                           "  capture string
                          0   push zero
                        ;!    skip `;` (end program)
                       $      drop zero
                     q@       reverse TOS
                 '43'         push 34 (")
                c             convert to char
               O              output all
              ;               end program

Befunge sees:

<@,+1!',k- ';';Oc'43'q@$;!0"
<                            move left
                           " capture string
                         !0  push 1
              ;         ;    skip this
         - ';'               push 27
       ,k                    output top 27 chars
   +1!'                      push 34 (")
  ,                          output "
 @                           end program

05AB1E/2sable, 14 bytes, non-competing

0"D34çý"D34çý

Try it online! (05AB1E)
Try it online! (2sable)

2sable is derived from 05AB1E and is similar, but has major differences.

Trailing newline.

Bash/GolfScript, 73

.~0 () 
{ 
    declare "-f" @* ".~0" " () 
"+@n.;
    echo '.~0;'
}
.~0;

There is a trailing space on each of the first 3 lines.

Bash/GolfScript, 78

alias :a~a.='eval "alias :a~a."\
;set [61 39]+a[39]+n"":a;echo ":a~a."'
:a~a.

CJam/GolfScript, 34 bytes

{"__X.0#@@?LL
;~"N}__X.0#@@?LL
;~

The byte count contains a trailing linefeed, since the program wouldn't be a quine without it.

While CJam and GolfScript are very similar in some aspects, there are a lot of differences. To make this an "honest" polyquine, I decided to rely on the differences as much as possible. Except for the block and string syntax (which the languages share with oh so many others), no part of the code achieves exactly the same in both languages.

The online GolfScript interpreter has a bug; this program works only with the official interpreter.

Example run

$ cat polyquine
{"__X.0#@@?LL
;~"N}__X.0#@@?LL
;~
$ md5sum polyquine <(cjam polyquine) <(golfscript polyquine)
e2f1f3cd68abbbceec58080513f98d9a  polyquine
e2f1f3cd68abbbceec58080513f98d9a  /dev/fd/63
e2f1f3cd68abbbceec58080513f98d9a  /dev/fd/62

How it works (CJam)

" Push that block.                                                                        ";

{"__X.0#@@?LL
;~"N}

" Push two copies of the block, 1 (computed as 1**0) and rotate the block copies on top.  ";

__X.0#@@

" If 1 is truthy (oh, the uncertainty), execute the first copy; else, execute the second.
  Evaluating the block pushes the string it contains; N pushes a linefeed.                ";

?

" Push two empty arrays.                                                                  ";

LL

" Discard one empty array and dump the second.                                            ";

;~

" (implicit) Print all items on the stack.                                                ";

How it works (GolfScript)

# Push that block.

{"__X.0#@@?LL
;~"N}

# Push a copy of the block; _ and X are noops, # initiates an inline comment.

__X.0#@@?LL

# Discard the 0 and execute the copy of the block.
# Evaluating the block pushes the string it contains; N is a noop.

;~

# (implicit) Print all items on the stack, followed by a linefeed.

CJam/GolfScript, 12 bytes

{"0$~"N}0$~

Cheaty solution that avoids the languages' differences as much as possible.

Try it online:

• CJam
• GolfScript

How it works (CJam)

 "0$~"       " Push that string.                                                          ";
      N      " Push a linefeed.                                                           ";
{      }0$~  " Push a copy of the block and execute it.                                   ";
             " (implicit) Print the stack.                                                ";

How it works (GolfScript)

 "0$~"       # Push that string.
      N      # Undefined token (noop).
{      }0$~  # Push a copy of the block and execute it.
             # (implicit) Print the stack, followed by a linefeed.

Bash/Ruby, 104 82

"tee`#";puts <<a*2+'a'#`" -<<'a';echo a
"tee`#";puts <<a*2+'a'#`" -<<'a';echo a
a

Older version:

"alias" "puts=f()(tee -;echo a);f"
puts <<a *2+"a"
"alias" "puts=f()(tee -;echo a);f"
puts <<a *2+"a"
a

Bash/Ruby, 128 without undefined behavior

"alias" 'puts=f()(a=`cat`;echo "$a
$a
a");f'
puts <<'a' *2+"a"
"alias" 'puts=f()(a=`cat`;echo "$a
$a
a");f'
puts <<'a' *2+"a"
a

Python 3 and JavaScript, 134 bytes

Here's my (final?) attempt:

a='eval(a.split(" ")[2%-4]),1//2# q=String.fromCharCode(39);console.log("a="+q+a+q+a.slice(-8)) print(a[-12:]%a) a=%r;eval(a)';eval(a)

It can probably be golfed a bit more, especially if anyone knows a better way to get single quotes in JavaScript.

Boiled down, the program looks like this:

a='a long string';eval(a)

The eval() function will evaluate expressions in both languages. So the long string gets executed:

eval(a.split(" ")[2%-4]),1//2# ... the rest gets commented out

This splits the long string by spaces and evaluates the substring indexed by 2%-4. JavaScript will run the third substring (2 % -4 == 2) and Python the second last (2 % -4 == -2), because their modulo operators behave differently for negatives.

The rest of the string gets ignored in both languages. JavaScript stops at the //, while Python sees it as integer division and stops at the #.

So JavaScript prints the source code to the console here:

q=String.fromCharCode(39);console.log("a="+q+a+q+a.slice(-8))

And Python here:

print(a[-12:]%a)

Both make use of the final part of the string, which is a template of the program:

a=%r;eval(a)

Ruby/Perl/PHP, 52

$b='$b=%c%s%c;printf$b,39,$b,39;';printf$b,39,$b,39;

Copied verbatim from Christopher Durr's Perl quine.

This is rules abuse. Ruby and Perl are definitely not the same language, nor is Perl a subset of Ruby (most of the linked Perl quines don't work in Ruby, for example). But Ruby was designed to be able to look a lot like Perl if you want it to, and this happens a lot when golfing.

PHP/Perl - 171

#<?PHP$s=1;$t="";
$a='%s<%cPHP$s=1;$t="";%c$a=%c%s%c;$t==$s?$t="#":$s;printf($a,$t,63,10,39,$a,39,10,63);%c#%c>';$t==$s?$t="#":$s;printf($a,$t,63,10,39,$a,39,10,63);
#?>

Run with:

$ php quine.pl
$ perl quine.pl

The php code is actually running (not just printing itself).

Ruby/Mathematica, 225 bytes

Here is my own very beatable polyquine (which serves as example and proof-of-concept):

s="s=%p;puts s%%s;#Print[StringReplace[s,{(f=FromCharacterCode)@{37,112}->ToString@InputForm@s,f@{37,37}->f@37}]]&@1";puts s%s;#Print[StringReplace[s,{(f=FromCharacterCode)@{37,112}->ToString@InputForm@s,f@{37,37}->f@37}]]&@1

The first part is based on this Ruby quine and is basically:

s="s=%p;puts s%%s;#MathematicaCode";puts s%s;#MathematicaCode

The string assignment is exactly the same in Mathematica. The puts s%s is interpreted as a product of 4 symbols: puts, the string s, % (the last REPL result or Out[0] if it's the first expression you evaluate) and another s. That's of course completely meaningless, but Mathematica doesn't care and ; suppresses any output, so this is just processed silently. Then # makes the rest of the line a comment for Ruby while Mathematica continues.

As for the Mathematica code, the largest part of it, is to simulate Ruby's format string processing without using any string literals. FromCharacterCode@{37,112} is %p and FromCharacterCode@{37,112} is %%. The former gets replaced with the string itself, (where InputForm adds the quotes) the latter with a single %. The result is Printed. The final catch is how to deal with that # at the front. This is Mathematica's symbol for the first argument of a pure (anonymous) function. So what we do is we make all of that a pure function by appending & and immediately invoke the function with argument 1. Prepending a 1 to a function call "multiplies" the result with 1, which Mathematica again just swallows regardless of what kind of thing is returned by the function.