As reported at https://www.wikidata.org/wiki/Wikidata:Contact_the_development_team#Not_possible_to_change_the_language_when_updating_monolingual_text
If you create a monolingual text statement in one language and save it.
And then try to change the language the save button is not enabled (unless you also change the text)
Can confirm. The issue is, in my opinion, that a change to the language is not detected as a change. A workaround (an ugly one, but it works) is to set the cursor to the language field and add a space at the end. This is detected as a change, and makes the save button work as expected.
Code to start looking at: ExpertExtender.LanguageSelector.js
Next step: git bisect