Sharing (characters) is Caring!

20 languages

Using  ()-#*.05;=>^Vefikloqrst (24 characters) so far:

let f l=floor(l**0.5);;       OCaml
let f l=floor(l**0.5)         F#
f(l)=floor(l^.5)              Julia
f(o)=floor(o^.5)              PARI/GP
s^+>5 0>5 0*.5l               Pyth
(* **.5).floor                Perl 6
floor.(**0.5)                 Haskell
flr.(tt ^.5)                  Wonder
l=>l**.5^0                    JavaScript
l->l**.5^0                    Cheddar
.5^5*5f                       Pyke
(isqrt)                       Maple
ri.5#i                        CJam
isqrt                         Common Lisp
0.5^k                         MATL
*.5^0                         Jelly
*.^0                          M
t0^                           05AB1E
q f                           Japt
r#                            Pushy
V-                            gs2

10 languages

Character set of 19 characters (in CP-1252):

,-/12@QUVX^fkmt¬÷␊␍

Where ␊ represents a line feed and the ␍ represents a carriage return.

(Script for checking the characters)

Jelly, 2 bytes

ƽ

Try it online!

05AB1E, 2 bytes

t1X-^

Try it online!

GS2, 2 bytes

V-

Try it online!

Jolf, 3 bytes

mfU

Try it here! (works best on Firefox)

Japt, 2 bytes

¬f

Try it here!

MATL, 3 bytes

X^k

Try it online!

Pyke, 3 bytes

,1f

Try it here!

Pyth, 6 bytes

/@Q2t2

Try it here!

Oasis, 6 bytes

1␊2/m1÷

Try it online!

2sable, 9 bytes

t1X-XX--^

Try it online!

5 languages

Gonna add more languages soon.

Character set (22 bytes):

s^Q.5=>*|0√Lrdmlab :/1

Pyth

s^Q.5

Javascript

Q=>Q**.5|0

Actually

√L

CJam

rdmQ

Python

lambda Q:Q**.5//1