Read Monolingual modes.xml

[mr-martian]

Every bilingual option except which transfer system to use can be derived from the monolingual modes.xml, so why not have a --with-lang1 and --with-lang2 that figures it out? This could also then support creating translators for languages with variants or multiple orthographies.

Some specific notes:

• if there's a mode matching *-lexc or *-lexd, we need HFST trimming
• are all preference files present in monolingual modes, or should we add an option for that?

[unhammer]

apertium-nno/modes.xml has nno-gener-prefs at least

https://github.com/apertium/apertium-nno/blob/master/modes.xml#L19-L29

For bilingual preferences, it's kind of independent of the existence of monolingual preferences. That is, people might potentially want preferences when translating into some language that doesn't have monolingual preferences; they might also not want to bother with bilingual preferences even if the monolingual has preferences (though I'm guessing this situation is less likely). Maybe a good default is to include bilingual prefs if there are monolingual, otherwise there's a switch --biprefs=on|off