How to change plug-ins language?

Quote from Plex Framework manual:

3.2.4 The Strings directory
The Strings directory contains JSON-formatted text files for each language the plug-in supports.
String files should have a .json extension and be named according to the ISO specification
for language codes (e.g. en.json). The name may also include an optional country
code (e.g. en-us.json).

So I made files en.json and ru.json for my plug-in, however changing Plex Web language to russian has no effect on my plug-in. Neither it works with other plug-ins like Schweizer-Fernsehen.bundle.

I tried checking Locale.CurrentLocale and Locale.DefaultLocale values, and got None and en-us respectively, shouldn’t Locale.CurrentLocale be ru or ru-ru?

Also checked Request.Headers for X-Plex-Language, got nothing.

Looks like it is a known Plex issue and temporary fix is available: Another localization question.