SYMPTOM:
The Administrator can manage the language settings for users per project by using the "User Language Preferences Manager" which is found via the Project Configuration > Language > User Preferences:

When a user changes the language settings in Web via the Web Preferences, no changes are recorded in the User Language Preferences Manager:

CAUSE:
This is a known issue in Strategy 9.x.
ACTION:
Strategy will continue to investigate and post updates as soon as they are available.