This fixes a bug where a keyboard didn't show up in the language dropdown
if the first language it specified wasn't a normalized language tag.
Previously in that case we added a custom keyboard with whatever language
tag the keyboard specified. With this change we now continue to use
the first language of the first keyboard but normalize it. Since the
selection of the language now happens a little earlier it is slightly
possible that there will be unexpected side effects that didn't show up
in my testing.
Fixes: #14465
This improves the implementation for normalizing language tags
when installing a keyboard. Based on the Windows implementation
in `Keyman.System.CanonicalLanguageCodeUtils.pas`.
One difference to the Windows implementation is that the mapping of
3- to 2-letter tags is based on `langtags.json` instead of
`iso639-3.tab`.
Closes#3399.
