Separate Client Profile Language from Active View Language
When a client changes their language, WHMCS updates the client's profile language, rather than just updating their current view.
The person who manages a client's account and who may want to view a specific page in a different language, may not necessarily want the whole profile's language to change (which would also impact email notifications, etc).
Instead, I would prefer decoupling the language of the view from the language of the client's profile. If, once logged in, the client switches their view's language to one that is different from their profile language, you can (in addition to just switching the language in the current session's view) pop-in a message or div telling the client that this language is different from their profile language, and asking them if they would also like to update their profile's language preferences (which they can simply click yes/no without breaking them out of their current view's context).