Forgive me, the last time I really was deep in Skaven-Lore was around 1995-2005 maybe (there is probably stuff I won't discuss because that lore might not have existed yet). This all popped into my head the other day for absolutely no reason.

What I do know about the Skaven Language:

1. THERE IS NO SUCH THING AS SKAVEN

2. The Language is called "Queek" or "Queekish"

3. Words are often repeated (especially exciting words)

4. The have a lot of different words for "Kill" (often compared to how the Innuit people have a lot of words for "Snow.")

Mechanically speaking, to make the rat-noises requires a lot of action with the tongue and incisors, and this would be no different really, for a human or a rat-man... ratmen have been heard to speak other languages, so they clearly have similar vocal folds and probably have guttural sounds as well, but its mostly in the nose, tongue, and front teeth with a variety of clicks, sucks, spits, and that "Tk" noise you can make putting your tongue against your incisors. They can probably just speak their own language much louder due to the long rat snoot being a better sound chamber. If any other (more human-head shaped head)-species were to try to converse in queekish, Skaven would probably think it sounds weird like we are whispering. When Skaven speak in human-head-shaped-head languages, screechy sounds would be more screechy, and guttural sounds would sound unskilled (due to lack of practice with using more precise guttural sounds).

As for the repetition of words, and the multiple words for "kill," I believe, that these things are linked due to a core mechanic of how the language it's self might be structured. Keep in mind that in lore, when reading the speech of skaven, or if skaven were to speak in some other language (but they would still have the logic of their own language ingrained in what they may choose to say in order to express themselves clearly). The language you speak naturally does determine some things like what you think is important information.

I'm also thinking about how Germans seem to have a lack of spacebar on their keyboard. Many German words seem (from my English speaking perspective) to just be two words without a space in-between, and I'm going to throw that in there as a part of it too.

Imagine a language, where you arent allowed to speak any single words. Each word must be paired with a word that increases its description.

First and foremost, the Verbs. A verb must be spoken in pair with a word that describes how the verb was done. This is not limited to adverbs, but can include nouns, or normal adjectives as well and also other verbs. This rule ESPECIALLY applies to the word kill.
You can Kill-stab, or Kill-choke, or any other such thing as this.
You can knife-kill, or spear-kill, or any other such thing as this.

But the other types of words must also be paired. Its impossible to use an adjective without at least a noun attached pre or suffix (now that I think of it, pre or suffix attachment might have to do with tense. The difference between kill-knife, and knife-killed, being like "I will kill" vs "I have killed" (with knife).

Also, metaphor vs. literal. In the case of a word actually repeating, it is being paired with itsself in order to really express the HARD definition of the one word.

"GrinkTikward! He-bastard hurt-stabbed me-self!"

"Stab-stab?"

"No-No, it-was Mind-stab :( "

TL;DR SKAVEN DO NOT EXIST