Fix language change

• Load the language files correctly when language is changed
• Save user language in localstorage so that we can have it even if disconnected (but still load it from user settings eventually since user might be on a different device)
• Fix pt-PT -> pt-BR
• Clean some obsolete config.exs comments

Later changes will allow to set the language without an account (see #375 (closed))

Closes #400 (closed) #405 (closed)