Replace Locale object with just strings

[rogthefrog]

The code is playing fast and loose with types for locales, and sometimes it's sending Locale objects and sometimes just strings, even when the type hint requests one type.

The Locale enum doesn't add much functionality, and it causes issues when the wrong type is passed (e.g. if the code expects a Locale but receives a string, or vice versa, an exception is thrown unless you handle it).

A string + maybe one validation function like is_locale(locale): return locale.lower() in ("en_us", "fr_fr") would simplify things.

@bkorycki @rogthefrog discussed this in Discord.

[rogthefrog]

Try again with named constants like EN_US and FR_FR

[rogthefrog]

#768