If my genre lies somewhere in the fantasy-fiction spectrum, is the use of a more "dated" narrative style/vocabulary going to narrow my audience to a harmful extent? I've always been drawn to the poetic, near-lyrical quality of old-fashioned writing (see above examples), and would like to incorporate it into my work(s). I think it adds an element of style, while fantasy works featuring exclusively modern voice and narration can sometimes seem bland or over-accessible. Granted, style/quality should not depend upon vocabulary, and "flowery" writing can become muddled, but working in such a style seems to offer me extra creative liberty.
What you describe is mostly what the genre of High Fantasy is about.
I have never found Le Guin or Tolkien to be archaic, or “dated”, it reads natural to me. I have more issues with Zelasny’s Princes of Amber series, or Moorcock's series, though.
Also some fantasy authors try to inject artificial “old style” and that is glaring and distracting to the narrative. The worse culprits try to mimic Shakespearian or pretend-medial “thou art…” type of diarrheic horse manure in dialogues.
Otherwise using “poetic, near-lyrical quality” is fine, it is bluring the line between litterature and fantasy and is quite common in the high-fantasy field.
I would say that if you enjoy that, and you want to include it into your work then you should do so.
I think the 'harmful extent' comes when it is over used, or it interferes with the flow of the story. But that is something that can be fixed in future edits.
If you are having conversations in some alternate language then provide some means of allowing the user to understand what is being said. Either through a direct translation or by allowing the reader to interpret the scene.
I would imagine the interesting aspects of that (for me at least) would be finding ways to convey the meanings of the other languages.
Some people will like how you write, some people won't. Some will think your inclusion of archaic voice is good, other's won't.
Be creative, write the story you want to write. You will enjoy writing it a lot more, and that will come across in the finished piece.