Thanks @osioke
I use latest Discourse version 2.9.0.beta1
( a1adc13125 )
And it was always like this (as far as I remember).
Here is a screen record of the issue:
It’s short and it shows how interface language became English instead of initial Russian (immediately after invitation accept).
It works like this with invitations accepted with Google/Facebook/Twitter/LinkedIn auth.
And with this issue every user needs to navigate and change their language back just after registration by invite.
Thanks!