Duplicate.

Examples:

separate(1200, "'") -> 1'200
separate(344400, "'") -> 3'440'400
separate(100, ",") -> 100 
separate(4334, ",") -> 4,334

looks weird because the right hand side is not entered as a string.
Try this:

separate(1200, "'") -> "1'200"
separate(344400, "'") -> "3'440'400"
separate(100, ",") -> "100"
separate(4334, ",") -> "4,334"