When we edit Wikipedia – or Wiktionary, or any other Wikimedia site – we use a long list of tools connected to MediaWiki, the platform our wikis use. These need to be translated, so editors understand what they do. This is very important.
This work is mainly done at translatewiki.net. You can create a user there and then pick a project you want to translate. There are several Wikimedia projects there. The most central ones are MediaWiki and MediaWiki (most important messages), but also for example Revision scoring, Wikimedia portals and Wikimedia Mobile Apps.
Sometimes new and urgent translations are requested on the Translator mailing list.
If you need help, you can ask for it here: