simplify locale handling by using lowercase strings everywhere

[rogthefrog]

The current locale implementation uses an enum, which is handy for validation, but it isn't used consistently throughout the code. Some functions declare a locale argument of the Locale type, but in actuality sometimes they get plain strings, so the function code has to 1) check the type 2) validate whether the string is a valid locale. The Locale object itself is just a bag of strings.

@bkorycki, @bollacker and I discussed simplifying and using strings instead.

[github-actions]

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