I think issue is related to something else. I have the same situation. I’m adding second locale for testing. Now when I’ve removed it, I do not have it in the list of languages, but it still prompts on the forms.
I thought about the settings in the database. I found that “journal_settings” table have three entries that sound similiar supportedFormLocales, supportedLocales, supportedSubmissionLocales, AND…
…only one of them is changing when I add/remove language - “supportedLocales”.
When I manually changed two others “supportedFormLocales”, "supportedSubmissionLocales"
and “Clear Data Caches” on Site Administration page, everything start working fine.
I thing it’s a bug