After quick googling I found out that this is a Hunspell dictionary format (aff and dic files).
I was able to set the language in Tools - Set language.
1) Is it possible to have Abiword auto detect the language?
2) Can I combine them when choosing a language? For example, a text that contains words from two (or more) languages. I guess my brute force way would be to add other languages to English dictionary files
as far as I can understand these are just text files with compiled words/patterns.
3) In the language list there are all maybe originally included languages. Can other languages that have no matching dictionary files be removed?