Improve support for handling locale information

[Martchus]

So far language/locale specifications are extracted from various formats/places as-is. That means with this information can not be dealt with in a format-independent manner. It would make sense to at least track the format being used. It would also be nice to add functions to convert between the formats like the library currently deals with the different character sets.

Annoyingly Matroska files have 2 ways to specify the language so having multiple native formats needs to be taken into account as well.

[Martchus]

The locale format is now tracked and it is possible to have multiple locales (with different formats) at the same time. The library does not allow to convert between formats so one still needs to take care to pass a locale in a format suitable for the current format.

[Martchus]

Further info about languages in Matroska: https://gitlab.com/mbunkus/mkvtoolnix/-/wikis/Languages-in-Matroska-and-MKVToolNix