A step forward in the internationalization of very common templates

Perhaps a first step is to draw a list of the words to be translated; such a list could be inspired from (a subset of) the Swadesh lists and from various translations on Meta and Commons.

Perhaps also it is a good idea of creating an extension similar to "Wikimedia messages" with very common words (called "CommonWords"?) with frequent words and an extensive qqq explanation to clearly define each word and avoid mistranslations.

What do you mean with "words"? Not lego messages I hope, perhaps most common single-word templates? I don't know if that's it. Probably the first thing to do would be a check of what templates have the biggest amount of interwikis across all projects.