Wikipedia has language editions, Wikidata has labels, aliases, descriptions, and some properties in multiple languages. This a great resource, to get the world’s knowledge in your language! But looking at the technical site, things become a little dim. Wikimedia sites have their interface translated in many languages, but beyond that, English rules supreme. Despite many requests, only few tools on Labs have a translatable (and translated) interface.
One exception is PetScan, which uses the i18n mechanism from its predecessor CatScan, namely a single wiki page on meta, which contains all interface translations. This works in principle, as the many translations there show, but it has several disadvantages, ranging from bespoke wikitext parsing, over load/rendering times on meta, to the fact that there is no easy way to answer the question “which of these keys have not been translated into Italian”? New software features require new interface strings, so the situation gets worse over time.
The answer I got when asking about good ways to translate interfaces is usually “just use TranslateWiki“, which IIRC is used for the official Wikimedia sites. This is a great project, with powerful applications, but I was looking for something more light-weight, both on the “add a translation” side, and the “how to use this in my tool” side.
ToolTranslate is a tool that allows everyone (after the usual OAuth ceremony) to provide translations for interface texts, in almost 300 languages. I even made a video demonstrating how easy it is to add translations (ToolTranslate uses its own mechanism, so the demo edit shows up live in the interface). You can even also your own tool, without having to jump through bureaucratic hurdles, just with the press of a button!
There is a simplistic demo page, mainly intended for tool authors, to see how it works in practice. Besides ToolTranslate itself, I also used it on my WikiLovesMonuments tool, to show that it is feasible to retrofit an existing tool. This took less than 10 minutes.