Language for languagetool should be taken from spelllang

The language for the languagetool spellcheck should be taken form `&spelllang`, or at least be configurable through another option. It should be quite simple in most cases; If spelllang has 2 characters, just pass them to languagetool. If it has 5, uppercase the last two.

Another variable `g:ale_languagetool_lang` could override the language, if user's spelllang is not supported by languagetool or differs from their vim spelllang. A Hashmap with languagecode mappings would be another option. Probably more reliable but a little more boilerplate. That is how I did it with textidote, when I adapted the existing PR for my own needs locally.

If using spellang seems to complicated, the variable `g:ale_languagetool_lang` would be much appreciated.

Start at the languagetool spellcheck entry point and inspect how its language is currently selected. Compare the requested spelllang-derived value with the proposed g:ale_languagetool_lang override or language-code mapping, then verify that the selected language is passed correctly and that unsupported or differing Vim spell languages remain configurable.

Written by the indexing model from the issue text.