How can I filter search results to a specific language?

It depends on the page:

• Subtitle search: open the "Languages" section in the left sidebar and click a language. Each entry shows how many videos match.
• Metadata search: use the "Auto Lang" dropdown to filter by the automatically detected language. You can also pick one or more languages in "Manual Subs" to find videos with manually added subtitles, captions or transcripts in those languages.