1. Using the Locale on API<21 doesn't make any sense because it's a no-op
anyway. Slightly restructured the code to avoid that.
2. API<21 often reports languages with non-standard underscores instead of
dashes. Normalize that too.
3. Some invalid language tags on API>21 get normalized to "und". Use original
tag in such a case.
Issue:#6153
PiperOrigin-RevId: 258773463
