[ENH] Add language to corpus

[PrimozGodec]

Issue

Implements the new approach to language detection in the add-on.

Description of changes

This is the first of a series of PRs connected to moving language setting to the corpus:

• The function for language detection and dict of supported languages (all languages supported by any method in Addon)
• Removing the old language_codes script, which is now replaced with the language module
• Adoption of corpus to store language setting. Corpus can have the language set to the iso code of the language or None
• Setting language attributes to repository datasets
• Implementing language detection and dropdown in Corpus viewer
• Temporarily copy attributes until it is fixed and released in Orange [FIX] Table - copy attributes when creating table from table orange3#6189

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #916 (9914fa7) into master (502eaf7) will increase coverage by 0.07%.
The diff coverage is 88.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #916      +/-   ##
==========================================
+ Coverage   77.18%   77.25%   +0.07%     
==========================================
  Files          86       86              
  Lines       12043    12113      +70     
  Branches     1578     1586       +8     
==========================================
+ Hits         9295     9358      +63     
- Misses       2447     2452       +5     
- Partials      301      303       +2     

[PrimozGodec]

It should be ready now. I also considered all comments from #874