I have a similar problem.
Sometimes, if I use English in wp-admin panel, then I’m not able to change language in the frontend.
I use Italian as default language. But in wp-admin the default language is English.
I noticed that if I log in to the admin panel, then in the frontend I’m not able to switch back from English to Italian.
I think that cookies are somehow conflicting.
Thank you, I really appreciate your plugin.
Regards