You can translate the LanguageTool user interface online at Transifex. The English strings are downloaded automatically from github. If a new string appears, the completion status of all languages will be less than 100% so you can see that something needs to be translated. See this help page at Transifex on how to get automatic notifications if there are new strings to be translated. Your changes at Transifex will not appear immediately in LanguageTool, though. If you have been making larger changes and want them to appear now, please contact Daniel (naber ..at.. danielnaber.de) directly.
As a developer, please also use Transifex and never commit to the property files directly (except the English one).
Whenever a new language is added, it needs to be added as a language at Transifex as well as in i18n_update.sh.
Our homepage has no real translations, but some languages have their own HTML/PHP page in git, in subdirectories at
community.languagetool.org again is translated via Transifex.