I've recently discovered that "Pelennor" is not Westron, but Sindarin.

Taken from tolkiengateway.net:

The name Pelennor translates to "fenced, encircled land" in Sindarin. Christopher Tolkien has noted that the first element derives from the Elvish root/element pel- ("go round, encircle"); the other elements appear to be end (from enedh "middle")' + (n-)dor ("land, dwelling"). The field was called by several other names as well, such as Fields of Pelennor, the Pelennor, and the townlands.

I did some minor investigating into the languages of Gondor on the same site and found this:

Sindarin had long ceased to be a "first language" in Gondor, but was learned in early youth (by those claiming Númenórean descent) from loremasters, and used by them as a mark of rank and high-blood. ... Westron became used more and more by the Dúnedain of Gondor themselves, so that at the time of the War of the Ring, Sindarin was known to only a small part of the peoples of Gondor (and spoken daily by fewer); they dwelt mostly in Minas Tirith and the adjacent townlands, and in the land of the tributary princes of Dol Amroth. Sindarin was used to be polite, especially in Minas Tirith.

Earlier, on the same page as the first quote:

After Minas Ithil had fallen and been renamed Minas Morgul, the Fields were walled by the great wall of Rammas Echor by Ecthelion II in T.A. 2954, to prevent an invasion. Presumably, the Fields took their name because of this enclosure.

So, all this considered, do the commonfolk of Gondor call the Pelennor Fields the "Pelennor Fields"? Or perhaps the "Rammas Echor Fields" (though unlikely, as this too is a Sindarin name [link])? Or do they not have a proper name for it and simply refer to the area as "the townlands"?