AniDB

In the advanced search I would also like to be able to search by subtitle language. That way when I do a category search I am not pulling up titles that have not been subbed. It really sucks to pull up a title sounds good to only find out that it's not released or subbed anywhere.

Example of searches I use to find good anime to watch next.

Elements: <genre>
Rating at least: <rating>

But no option to select subtitle language at all so missing an important search option when searching for my next anime to watch.

My 2 cents:
As I see it, subtitle language is NOT a part of an anime info, it's a part of a certain release. I doubt anime search "has the access" to the data about releases, besides I guess it's not that easy to decide if anime is completely subbed or not (well, it might be but it would probably result into more load on server, assuming this information is not precomputed and cached somewhere already).
But specifying some criteria for "Votes" and "Rating" should find pretty much only the anime that are completely subbed or the sub is close to the completion. If an anime has finished some time ago and it hasn't been
subbed/ripped in its entirety, it's probably not that great (same goes for ongoing anime that weren't picked by any group). Of course, such a search won't list ongoing series, but from their nature they can't be considered "completely subbed" anyway.
Well, a checkbox on search page like "Last episode is subbed" or "All episodes are subbed" (even when only from combined releases from several groups; but what about e.g. those very few anime with non-Japanese audio only like Highlander: TSFV or dubbed TV rips of old anime?) might be somewhat useful and it would require only one extra boolean information about the anime to store (and update). Of course, if it had to be done for every possible language, not only for English ...

Well we know their is a field for sub language. It's in the data so a search box where we can manually type in english, japanese, ect will give us this search capability. I know their is some series that are never finished, dropped, ect but this will allow us to find new series that are subbed to possibly watch.