After reading https://bugs.freedesktop.org/show_bug.cgi?id=37919 and some searching, I found out it was the "font" tab that needs to be selected instead of the "background" tab if you click on language -> "more".
This is a bug since it's not clear for the user what to do. Changing the language was the goal, not the background color. So please change the tab and everything is fine.
Thanks, already reported as "Bug 39793 - UI · FORMATTING: 'Set Language for Paragraph/Selection > More...' should always open the 'Font' tab (dialog 'Character')".
*** This bug has been marked as a duplicate of bug 39793 ***