Vome is available in English and French, and there are two ways to switch.
The difference matters: the globe icon in the top bar changes the language you are looking at right now, while Settings saves the choice to your account. If you keep finding Vome back in the other language after you log in, that is why.
Click the globe icon in the top bar, then pick English or Français.
The page changes immediately. There is no confirmation step to click through.
Go to Settings then Preferred Language then Content Language, and choose from the Select Language dropdown.
This one is saved against your account, so it is the language you get the next time you sign in, and on any device you sign in from.

Yes. The login page has its own language switcher, so you can read the sign-in screen in your own language.
Choosing it there also records the preference on your account once you are signed in, which makes it a reasonable place to set it if you are logging in for the first time.
English and French. Those are the two.
No. It is your own preference and nobody else sees the effect of it.
Each person on your team, and every volunteer, sets their own. An administrator cannot switch a volunteer's language for them from the database, so if someone is reading Vome in the wrong language, the fix is for them to change it in their own settings.
The Vome interface, and the support material that goes with it:
What it does not translate is content your own organization wrote. An opportunity description, a form question or a sequence step appears exactly as the person who wrote it typed it, in whatever language that was.