POEDIT
Free / Low Cost windows software which can scan your code and then provides all the translation features you need. Seems pretty popular.
WPML
Very popular, very featured, apparently a really good choice its hard to go wrong with and not excessively expensive.
Polylang
Another very popular wordpress choice
Weglot
Different to WPML and Polylang, aimed much more at auto translation of sites and non expert users.