'lang' is just a parameter of the media type. You can use the mime package to easily specify the language:

mediatype := mime.FormatMediaType("text/gemini", map[string]string{
	"lang": "en-US",
})
w.SetMediaType(mediatype)

By the way, you can use the ~adnano/go-gemini-devel mailing list for general discussion like this.

Ah, thanks. I'll use the mailing list in the future -- still getting used to what should go in the tracker vs mailing list on sr.ht