internationalization of interface messages in translate extension

Not that I know. You could reverse engineer the parser function that implements it in core, and try to make an extension that does what you like. We haven't done that before, though.

Okay thanks. I guess that to do it will be way too advanced for me! I'm testing more to learn how to make templates now, so maybe I'll figure out something. Two more question: (1) Is there a code that I can use in templates that will give me the language code of the visitors interface? Maybe I can use it to make the template do what I want. (2) Is there a code that I can use when I make a form, that will give me the drop-down menu with the list of all the languages used in the translate extension? The drop-down menu used to choose what language I will translate to. So I can make people add a correct language code to a value in the form.

For 1) see https://bugzilla.wikimedia.org/show_bug.cgi?id=2085 (the answer is: not by default). For the other parts, I think the answer is also no.

I assume you have already read https://www.mediawiki.org/wiki/Special:MyLanguage/Help:Extension:Translate/Unstructured_element_translation and https://www.mediawiki.org/wiki/Help:Extension:Translate/Page_translation_administration which provide some hints how to handle content translation with what is currently available.

Thanks for the info! For (1) I've found a template [1] that had what I needed, but need to make a landing page to make people able to find their language.

I've read this information yes. I understand that what I asked for is not a part of the Translate extension, just wanted to ask if there is a smooth solution, as you probably have good overview of what is possible to do. My best option now I guess is to make my own drop-down list [2]