Detecting the setting of the browser of the visitor is not enough. I think we can do better. Look at https://translate.google.com/, they are detecting the language with the text which was typed.
Topic on Talk:Wikipedia.org Portal A/B testing
Hello, we actually have another ticket for this type of work: https://phabricator.wikimedia.org/T129627 To be able to tell what language the user is typing in and to "automagically" switch to search in that language. Stay tuned!