I understand, that I give you the answer two months later. But maybe the answer will be read.
Just the texts of the menus and things like that are enough for me
The texts of the menus are here
(Dune folder)/mods/common/languages/
in file
en.ftl
Bad solution but it works: you can change text right in this file.
(Make reserve copy first)
Developers said (last year at least, in this forum) that user still
cannot create own language file, for example
sp.ftl.
It does not work. I tried.