The help text for the default language option contains a reference to an "Allow user to change language" option:
Assign the default language for the site.
When the Allow user to change language option is set, users can override this setting through their browser's preferred language configuration, configuration options on their Account page, or through links or buttons on most webtrees pages. (Transl. text 1329)
However, I can't find that option anywhere, nor any text in the translation material with that text.
I know I've read somewhere what to do if problems with the English text is encountered; I can't remember what it said, though, and I can't find it again, so I'll post it here.