Language names

[Lawrence37]

See #6657 for context.

This displays language names in their own language, e.g. Castellano Spanish is "Español (Castellano)". The name is defined in the language's respective file as a metadata variable called LANGUAGE_DISPLAY_NAME. I describe metadata variables in the commit message:

Metadata variables are defined in the first section of a language file,
i.e. the comments at the beginning of the file. At most one variable can
be defined per line. The definition takes the following format:
   #<header><whitespace>@<key>=<value>
where:
   <header> is any sequence of non-whitespace characters
   <whitespace> is any non-empty sequence of whitespace characters
   <key> is the metadata key name
   <value> is the metadata value
For example,
   #101 @LANGUAGE_DISPLAY_NAME=English (US)
defines a metadata key "LANGUAGE_DISPLAY_NAME" with value
"English (US)". Lines that do not match the format are ignored.

These changes only affect the names displayed in the drop-down for selecting the language. options still saves the language file's name. All language names have been added to the best of my ability.

Screenshot:

Excuse the tofu for Chinese. Something is wrong with the fonts on my system.

Closes #6657.

[Hombre57]

@Lawrence37 PR tested on my side, it's working fine 👍 . This PR should be merged once the README has been updated.

[Lawrence37]

Thanks for reviewing, @Hombre57. I agree with the README.

[kmilos]

All language names have been added to the best of my ability.

Just in case you were not aware, you can pull these from https://cldr.unicode.org/

[Lawrence37]

Checking against CLDR, I confirmed all languages except for Chinese (Simplified) (not present in CLDR), Czech (čeština in CLDR, Český on my KDE system), and Brazilian Portuguese (not present in CLDR). Čeština seems correct based on a quick internet search.