Thanks for your contribution.
I believe translations are a work in progress rather than an implemented feature. If you look through the commits you will see that their are a large number of commits since the last release of the application and I believe that when the next release comes the majority of languages will then be supported.
You can see the below GitHub issue for further information about the current progress of translations:
https://github.com/LonamiWebs/Stringlate/issues/141
You can also see that the commits since the last release below which shows the translations that I believe will go to the next build:
https://github.com/LonamiWebs/Stringlate/compare/v0.12...master
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