Sorry to hear you’re having issues Tom. The plugin only changes the currently used admin language based on a cookie. Please make sure you are not using an application firewall like Cloudflare that might block cookies.
The problem is, that I’ve got a number of sites using your plugin. Some of them change admin language and some do not. I use the same computer so it is the same firewall and browser. Though the behavior is still the same. Some of websites change language according to plugin command but some of them do not.