I don't agree with Haba on the English thing but Tolkien based his fictional languages on real ones, precisely because they just sound "right" to our ears, they resonate. So the high Elves are Finns, Wood Elves are Welsh, dwarves are underground Jews, horse masters are Saxons and so on. Even if you're not a linguist it just somehow, subconsciously makes sense.
Oh man, back in the 1990s I used to be active on rec.arts.books.tolkien, we talked a lot about this shit.
Tolkien's conlangs aren't really based on any real-world languages, although they are influenced by them. High Elven is phonetically and structurally somewhat similar to Finnish, but there isn't anything a Finn would recognise there – "Aï, laurië lantar lassi surinen / Yeni unótimë ve ramar aldaron" doesn't look or sound at all like Finnish to a Finn, diphthongs notwithstanding. The Finnish cognates are really few and far between, only one I can think of off the bat is "ilmen" for the heavens which is a cognate for "ilma," air, probably by way of Ilmatar, the Kalevalan Lady of the Heavens.
Sindarin is even less influenced by Celtic languages. It is descended from Noldorin the same way modern English is descended from Old English. "A Elbereth gilthoniel / silivren penna miriel" doesn't sound Welsh or Irish or Gaelic to any Celt.
As to the Rohirrim, in LotR Tolkien uses Old English as a stand-in for their language, the same way he uses modern English as the stand-in for Westron, which is the common language in the West of Middle-Earth where it is set. The reason is that the relationship between Rohirric and Westron is similar to Old and modern English, so a Westron-speaker would get a similar "feel" from Rohirric as an English-speaker would get from Old English. Hobbit placenames are derived from Middle English, reflecting the common linguistic roots again – hobbit names are actually "translated" or "transposed" from their language into Middle/modern English. IIRC Meriadoc Brandybuck's real name is something like Kalimak Braldagamba for example – Westron really isn't at all like English. And yes he did go a long way to constructing these languages too, even though they don't even appear in the books.
Tolkien used languages beautifully to manage the sense of familiarity and strangeness in his world. He would use his conlangs as-is if they would have been alien/different enough for Westron-speakers, hence Sindarin, Noldorin, Khazâd, and Black Speech are left untranslated, but Westron becomes English, Rohirric becomes Old English, and hobbit placenames like Michel Delving become Middle/Old English.
Languages that are just completely made up, like blaa-bli-blaa baby speech, do not have this effect. Usually they sound incredibly lame.
Yep, agreed. It's a tricky proposition. You don't need to have fully fleshed-out conlangs though, you can substitute real-world languages or even just phonemes from real-world languages if you're clever about it, the point is that there has to be a purpose to it. If it's just random it's stupid. Robert E. Howard for example didn't create full conlangs but he did use phonemes from real (dead and existing) languages in his place and character names and occasionally in the text too. That worked quite well because he was thoughtful about it. If in his cosmology Gypsies descended from a particular civilisation, Zingara suggests that quite nicely, like Khem and Thoth-Amon suggest Egyptians.