In the Github repo, find the translation folder, look at the English (US) file, look at the German one for a pretty-much-always-up-to-date translation, and then make the Sinhala one following the same pattern. Basically the Nth line in each file is that language’s version of the same text. Make a branch for your translation and then file a pull request.
If you’re not into Github, ummm, find someone who is, I guess.