When a user attempts to access login forms in a language other than English, we get the English form instead of the expected language. We can't seem to make this work for other languages.
Customer was testing French language exclusively with Chrome. Instead of sending the expected 'fr-FR' as the Accept-Language header value for French, Chrome was sending 'fr'. This did not match any locales in the configuration and caused the request to default to English.
Updating the DefaultLocale Agent Configuration Object parameter to include 'fr' along with making a 'fr' copy of the 'fr-FR' forms folder (also requires an update to the file names within the folder) enabled French forms to be served for all browsers. This had to be repeated for other languages. The key is to study an http trace from the browser type and language to be supported, making sure there is an existing forms folder for that value and that the value is added to the DefaultLocale ACO parameter.