This is for SharePoint 2010.
We have added a custom field to our People Search results called Foreign Languages. If you search for a person, the value shows up in the results. However, if you try to search by those values, say "Russian", no results are displayed.
I believe I have all the necessary options checked for it:
Metadata Properties>Edit Managed Property>Property>Allow this property to be used in scopes
Metadata Properties>Edit Crawled Property>Property>Include values for this property in the search index
We run a full crawl every Sunday.
I think that's it; I'll be more than happy to provide more information if needed. I'm honestly not sure what else to try or check.