[Huinesoron]

Originally Posted by William Cloud Hicklin

Because - all discussions of Appendix E aside - Isengard is definitely not Sindarin, nor is it Westron. It's Rohirric, that is, Old English.

But going straight back to Appendix E, is it Old English or modernised Old English? The note at the end of part II starts:

So also in the case of the personal and place-names of Rohan (where they have not been modernised), except that here éa and éo are diphthongs...

The sounds he names are from the Old English "translated" forms: Éomer's Rohirric name does not begin éo. So he's distinguishing here between Old English forms and modernised Old English.

The modernized forms are easily recognised and are intended to be pronounced as in English. They are mostly place-names: as Dunharrow (for Dúnharg), except Shadowfax and Wormrongue.

Appendix F calls out Snowbourn as a fourth word where he "modernised the forms and spelling", and cites Edoras as one he didn't. It also explains the rationale:

... I have followed the Hobbits. They altered the names they heard in the same way, if they were made of elements that they recognised, or if they resembled place-names in the Shire; but many they left alone.

I would argue that "Isen", specifically, counts as an "element that they recognised", because it appears in the name of Pippin's direct ancestor: Thain Isengrim II. Pippin, Merry, and Frodo actually share two great-uncles whose names use "Isen-": Thain Isengrim III, and his youngest brother, Isengar, who went to Sea in his youth.

On the basis of Appendix F, "Isengard" is a perfect example of a name Merry and/or Pippin would have "modernised", which - per Appendix E - means it would be pronounced as if it were modern English.

hS