I have a Firebase hosted website that contains an index_fr.html and index_nl.html.
How can I create a rewrite rules that uses the user agent language to serve the correct index_nl or index_fr page (without using Firebase functions)?
Update August 12th, 2020: Firebase Hosting now supports i18n internationalization.
Firebase Hosting rewrites are based on the requested path only. There is not support for such conditional rewrites in the Firebase Hosting configuration.
You might want to file a feature request for it.