Support multi-language documents

[matze-dd]

A way with -- hopefully -- only moderate intrusion could be as follows.

YaLafi core

• Macros for language change as \selectlanguage leave special tokens that indicate the switch of language in the expanded token list.
• The parser switches language-dependent settings as maths replacements, when it sees such a special token.
• Currently, the function utils.get_txt_pos() transforms the expanded token list into a string and a character position map at the very end. This is changed. It then produces separate strings and position maps for each detected language, using the special tokens from above.
• Altogether, this should even allow nested language changes and changes, e.g., inside of \text in equations.

Yalafi.shell

• It sends multiple requests to LanguageTool.
• As currently, messages are sorted according to their occurrence in the LaTeX text.

We should implement some heuristic that decides whether a language change by \foreignlanguage really breaks the text flow of the surrounding language, or whether it is rather short. In the latter case, it should be substituted by a placeholder in the text flow of the surrounding language (as is done with inline formulas), continuing its sentence / paragraph.

There will remain at least one bug. The scanner currently is initialised with a language code that, for instance on 'de', detects "' as a special token in German texts. Since the scanner first scans the whole LaTeX text, this won't be changed inside of \foreignlanguage{english}{"'}.