Write a conversion function that maps lang_id to human-readable name

[dmitry-kabanov]

In error messages, it is useful to output the actual human-readable language name.
Write now, only integer identifiers are printed, which is not human-friendly.

Write an auxiliary function/hashmap, that maps the language IDs to the language names.

[dmitry-kabanov]

I did this today to exercise a bit of programming while sailing in the ocean of LaTeX in the PR #720