Hi Mike,
We added a "language" property to the feed info a few weeks back. This is currently returned by /v3/feeds and by /v3/search/feeds. E.g. this will return feeds with "language":"fr" for feeds from the French newspaper Le Monde:
And the feed info endpoint will also report this feed as French:
The value is based on both the reported locale from the feed itself, and from a language detection on the feed content itself.
There is also a "locale" parameter to the /v3/search/feeds endpoint, which will "hint" the search engine to return results in this locale (it is not a filter!). E.g.:
will return (more) results in Japanese about Amazon.
Hope this helps.