Navigators send a list of acceptable languages (English, French etc.) by decreasing preferences. By default, MediaWiki on commons and other multilingual
sites should default to the first language that it knows in that list.
Francophone users have complained that the commons user interface is in English even though they follow links from fr.
The UniversalLanguageSelector extension can do this, but it is not enabled for anonymous users due to caching issues. See T58464: Allow anonymous users to change interface language on Wikimedia wikis with ULS as interim step towards this.