Read Monolingual modes.xml

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?

Start by locating the command-line option handling and the code that reads monolingual modes.xml. Determine how existing bilingual setup identifies transfer systems, handles *-lexc or *-lexd modes and HFST trimming, and checks preference files. Done should include working --with-lang1 and --with-lang2 support for variants or multiple orthographies, with the preference-file behavior resolved.

Written by the indexing model from the issue text.