In the admin console make sure the field is filterable and searchable, and it should allow it show up in the people filter set. Hope that helps
You can't search a particular field as far as I know. When talking to the search team they've explained that all profile fields that are searchable are concatenated to a single string. So search is unaware of the field a particular word originated from.
But what about doing a Full search filtered to people and entering a language like "polish" - would that word be likely to often appear in other fields?
Otherwise a download in CMR of Profile Completion would give you an XLS you could filter on, even custom fields.
We have added extra fields that are multi select like experiences and training completed. These are searchable through adding a filter for the given field in the People Search.