Although your version is still 2.4.x your issue is quite similar to this thread:
I think you could deal with it in a same approch:
Check if you delete OJS cache after value changing;
Confirm you have right permission settings to your cache folder in your server.
At last you may check inside your journal settings cache files – cache/fc-journalSettings-*.xml – just to confirm which values are set to supportedSubmissionLocales parameters.
Regards,
Israel Cefrin
Public Knowledge Project Team