I am trying to change the language of the user interface (to German) in order to have words in reports being rendered in that language. The operating system (Ubuntu) is set to English.
~ export LC_COLLATE="de_DE.UTF-8@collation=standard"
~ gramps
(gramps:122539): Gtk-WARNING **: 10:11:35.932: Locale not supported by C library.
Using the fallback 'C' locale.
and put an ampersand behind that if you want to be able to type more commands in terminal.
When I do this, I see the same error message, but my Gramps does speak German.
If you installed ubuntu in English only, you may need to install the German language pack first.
Update:
When I use the instructions from the wiki, Gramps keeps speaking Dutch to me, and that is quite logical, because this collate thing is for sorting only, not for the UI language. The wiki mentions that, but should be updated to give the proper instructions.
When you follow my example, the export is not needed, because the LANG variable only affects the command behind it, on the same line. When you do an export, all other commands that you give in terminal will also be affected.