[ENH] Use ISO language setting in widgets

[PrimozGodec]

Issue

In the beginning, I was keeping language names in widget settings. It turned out it is better to keep languages as ISO codes since language names can change in the future (like we did with Ancient Greek), but ISO codes will probably not.

This

Description of changes

• It should fix the issue reported in [ENH] UDPIPE - Use ISO language codes  #1030
• Change the Ancient greek language to Ancient Greek to be compatible with other languages (and compatible with ISO standards).
• Keep languages as ISO codes in settings to bypass problems with language name changes and easier management.

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #1034 (c6a7082) into master (e7c360d) will increase coverage by 0.23%.
The diff coverage is 92.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1034      +/-   ##
==========================================
+ Coverage   82.18%   82.42%   +0.23%     
==========================================
  Files          92       92              
  Lines       12283    12322      +39     
  Branches     1670     1678       +8     
==========================================
+ Hits        10095    10156      +61     
+ Misses       1880     1855      -25     
- Partials      308      311       +3