What's the recommendation if the audio language isn't on the allowable value list.  I bought my first Ukrainian Blu-ray today, and that language isn't available.  I've seen subtitle language not represented, so I just skip that, but this is the only audio language on the disc.  I know that "Other" is an option (and that's what I'll choose), but do people try to mention it elsewhere (without breaking any rules)?

Mentioning it anywhere else without breaking the rules is impossible. "Other features" is obviously not a correct field for it. I don't think it NEEDS to be mentioned. The advantage of the dedicated field is that you can filter/search for it via the UI. Without that, you only have informational text (yeah, you can search "other features", but are you going to search for Ukrainian there?). That "provide the information" goal is already achieved by the back cover scan though. Anyone seeing "Other" in the list will check there.

So I don't think it's worth breaking the rules for it. Locally you can of course add it to Other features, but personally, I'd prefer the notes field. In fact, I have an "Original Language" tag with subtags for all languages, so that's where I would track it.