Well, if you think about it, switching can only work if you define “en” as generalOptionsLanguage in the French site.txt file and vice versa. That’s because the value is taken from the default language.
Edit: I think what I would try to do is to create a dashboard widget that saves the language configuration to an external file and then read that file in the config.
But the safer way would be to give the users a preconfigured installation instead if trying to automate it. After all, you don’t know in advance, how many and which languages they need, which locales of a given language they want to use etc.