On Windows, the plugin metadata always follows the current region setting and any language set for the application is ignored.
The reason seems to be that QLocale is not changed even if the LANGUAGE env is set. To work around this we can read the LANGUAGE env variable and use that if it is set in stead of QLocale::name().
I'm not 100% sure if this is the right solution, but this at least makes the plugin configuration page in Kate use the same language as the rest of the application if the language is manually sett.