Many very good explanations have already been provided here.
To address it again in a comprehensive manner:
Tolkien's proper names are to a certain extent translations from the "Red Book of Westmarch".
Westron as the language of the
Hobbits (and others) was completely translated into English. Proper names of the Hobbits, which are not 100% Westron, have also been translated into analogous language cognates. Before coming to
Eriador, the Hobbits used to speak the language of the people of
Rhovanion, some of whose descendants are the
Rohirrim. This ancient language, however, is related to Westron, just as the people of Rhovanion are related to the people who became the
Dunedain and thus strongly influenced Westron (to summarize briefly).
For this reason, Tolkien rendered the language of the Rohirrim with Old English and the language of the people of Rhovanion, ancestors of the Rohirrim, with Gothic terms.
Thus Tolkien
places the
languages Westron <- language of Rohan <- languages from Rhovanion in a similar naturally evolved, language-related context as
modern English <- Old English <- Old Germanic languages such as Gothic. Something similar is also evident, for example, in the (human) names of the
dwarves from the
Hobbit, which are borrowed from Norse (see the
Edda), since the people of
Dale and the surrounding area were strongly related to the people of Rhovanion, much as the Scandinavians were to the Germanic tribes on the European continent.
So you always have to keep in mind if the names and words are the actual language of
Arda, or a kind of translation by Tolkien to show language affinities, which was Tolkien's real purpose behind all those stories!





In my opinion, this is a very complex topic, and one should definitely meditate on the explanations about it in the appendices to
The Lord of the Rings as a basis. Since the subject (languages and proper names) was enormously important to Tolkien, you should definitely read up on it, even if you might not be so focused on languages.
So you always have to look at the names on at least two levels. Are they the original ones, or are they translations or adaptations? With the Elven languages, on the other hand, you can be sure that they are the "original".
(But it has to be said that
Sindarin was influenced by Welsh and
Quenya by Finnish and Latin.

)
In that "language translation chain" Tolkien was also not completely consistent: The proper names from the
Adunaic, that is the language of the Numenorians, seem to be reproduced in their original, although the Adunaic is very close to the Westron.
But that may also have something to do with the fact that Adunaic also contains many elements from
Elvish languages.
In addition, the
Numenoreans and Dunedain generally named their place names and often also personal names in "
Elvish".
Incidentally, Tolkien apparently also attached importance to the fact that this process was continued in many translations from English into the respective national language. For example, the typical English place names were also analogously adapted to many European place names, with the goal of making them seem as common and ordinary as possible, so that the reader would immediately feel somehow at home in
the Shire, no matter where he came from.
As far as I know, Tolkien, for example, was also relatively good at German (although more of its linguistic precursors) and contributed to the corresponding translations of the place names or at least validated them. (And the proper names of the Rohirrim didn't necessarily have to be adapted further here, as Old English shares many similarities with Old High German words.) I suspect, however, that this was not carried out so consistently in every language...
But to return to the
troll names: The Hobbit should perhaps additionally be looked at again as a particularly filtered retelling from the Red Book, namely in its translation as a book for children, although perhaps
Bilbo originally described it in a much more realistic and factual way? Who knows, but perhaps Bilbo already had fun recording his experiences rather in fairy tale form? Anyway, the names of
the trolls were "in truth" probably not known.
It is also possible that the trolls could only converse in a primitive form of Westron and therefore gave each other very common Westron names. (And these have been translated into our languages accordingly.)
In any case, these should just be typical village idiot names.
Tolkien language experts on the forum, feel free to correct me if I'm wrong somewhere. I may be advanced on the subject, but not an expert yet.
Edit: To someone who has only seen the movies so far, this probably all sounds completely idiotic. But it isn't, on the contrary! 😜
Tolkien was a linguist, and all of his motivation stemmed from that in origin.
Also, you can be sure that EVERY personal name, place name, and "fictional" language is carefully thought out and makes sense one way or another. Because his love of language is at the heart of it all.
By the way, my various nicknames have already been identified as "
trolls" in various internet forums. (Weird.) I've never really understood what that means though. 😇
But the bottom line is: Any name can be a troll name today! YOURS TOO!
