Another thing to note is the "boiler plate" text within the forms themselves.

Certainly the traditional "professional" forms interface will require the relevant language-specific versions of the forms to be installed and compiled as the text is hard-coded within the form. Most forms sit within relevant language subdirectories under the product tops.
So for "english", the forms sit under a US subdirectory.
So if you wish to have french text in the GUI then you will need the french application forms installed.
As mentioned by other respondents, the language setting then becomes a user profile, so one person can view the french forms and another the english forms etc. I've seen this done in Germany.

This may be different for self-service forms however, as the text is rendered and so can go through a NLS translation process before being presented - i've not done too much with other languages in SSHR so cannot be certain on this fact.