transifex - It’s the thing people like Tor and Tails and lots of others are using to localize their applications.
Apparently there is even a community of volunteers to translate stuff. Can translate application strings as well as the website.
(However, applications and website need to be prepared for website first. Apart from this programming work it takes also organizational work with transifex.)
Have looked into this (and many other) 3rd Party translation utilities in the past. Sadly their markups are quiet massive: Transifex Pricing | Plans that scale with your localization needs
Adding to that, Tails’s Transifex seems to be either broken or not available any more: https://tails.boum.org/contribute/how/translate/with_Transifex/
Have a nice day,