Support specifying the target language of an individual utterance

Support specifying the target language of an individual utterance.

```
# Menominee
\txn  pemētaehkipew
\m    paemet‑   ‑aehkw‑ ‑ape
\gl   crosswise face    sit
\wlt  initial   medial  final
\lg   mez
```

Should be a valid IETF language tag.

This is an add-on line code, meaning that if it has a line code, it doesn't require the other lines to also have line codes.