I use navigator.language which can return values likes "en", "en-US", "fr", "fr-FR", "es-ES", etc.

It seems calling setLang using the 2 letter value like "fr" works, but not the 5 letter value like "fr-FR". There should be a fallback from "fr-FR" to "fr", "es-ES" to "es", ...Basically just keeping the 1st 2 letters should fix it.