The root cause of this issue is the order in which the various MantisBT APIs are required in core.php. Basically, at the time the installed plugins are initialized, auth_is_user_authenticated() function required by the language subsystem has not yet been declared so the system falls back to default (English).

This problem has been resolved in 1.3.x by the new require_api system introduced by dhx, and the issue is no longer reproducible there.

I'm therefore marking this as resolved, and due to the complexity of the changes and to avoid introducing regressions, I will not backport the changes to 1.2.x.

If this is important to you, as a workaround, feel free to mess around with the order in which the APIs are included in core.php - basically I guess you'd need to move authentication_api.php before database_api.php but there might be additional, cascading dependencies.

This is minor issue and I agree that it is not worth investing so much time, especially if it is already fixed in 1.3.x. Thank you.