[hrktb]

I think it could be anything making sense (the default for a german company could be german for instance) as long as switching away is smooth and discoverable. People not familiar with the english alphabet for instance could be lost in the site, getting overwhelmed by the unknow information, even if they know the word "english" or "language". For people like in that case, the page could be in french it wouldn't make much difference.

As a visual marker for language switching I imagined having a flag, but looking at the replies, that seems non optimal.

The best behavior could be a popup shown only to users who's accepted languages don't match the current language, and keep the choice in a cookie perhaps ?