Prelude> putStrLn "\3619\3657\3634\3609\3648\3592\3657\3648\3621\3657\3591"
ร้านเจ้เล้ง

Note that you don't actually have the string "\3619\3657\3634\3609\3648\3592\3657\3648\3621\3657\3591" – rather, you have the UTF-32 string ร้านเจ้เล้ง, for which "\3619\3657..." happens to be a ASCII-compliant literal. By default, GHCi uses the Show instance to display results, which doesn't so much show things as spit out literals that can be used as Haskell code for the thing. It's conservative in terms of unicode. That's why

Prelude> "ร้านเจ้เล้ง"
"\3619\3657\3634\3609\3648\3592\3657\3648\3621\3657\3591"

On the other hand, the putStrLn, putChar, hPutStr etc. functions will just dump the string itself in UTF-8 rather than an ASCII-safe representation thereof.

If you're actually reading the escaped string from a file or something, you can simply read it:

Prelude> s <‌- getLine
"\3619\3657\3634\3609\3648\3592\3657\3648\3621\3657\3591"
Prelude> s
"\"\\3619\\3657\\3634\\3609\\3648\\3592\\3657\\3648\\3621\\3657\\3591\""
^{-- Note double escaping, because I'm showing a string that contains a string literal.}
Prelude> putStrLn $ read s
ร้านเจ้เล้ง