The problem is that High Mordentish, for all intent and purpose, is modern French whereas Low Mordentish is either Middle English or Anglo-saxon (can't remember). The words used are -very- different. You can check the Gazetteer III primers for suggested words.
It's obvious that for Mordentish, they tried to replicate the effect of the Norman invasion on England and its impact on language (the French language spoken by the nobility and the English language spoken by the masses at first) and how they became one. Obviously, Dementlieu didn't invade Mordent.
edit: Also, I think one of the major causes of confusion is that two different writers worked on Dementlieu and Mordent and clearly they had a different interpretation of what Mordentish was. In the Mordent entry, there is no mention at all of Low/High Mordentish, whereas in the Dementlieu (and Verbrek) entry, they clearly separate the two as two dialects.
Here's what Gaz III says about the use of Mordentish in Dementlieu and Mordent:
Dementlieuse use of Mordentish:
Mordentish is the primary language of Dementlieu. It is a curious tongue, in that it has two quite distinct dialects. I could find no particular explanation for the two diverse dialects and can only assume that they are the result of some long forgotten merging of two separate languages. In total, the language is very flexible, with a wide vocabulary. Generally, class defines the use of the two dialects: High Mordentish is spoken by the upper classes, while Low Mordentish is reserved for peasants and farmers.
High Mordentish is a very soft language, forsaking the harsh consonants of a language such as Lamordian for rolling vowels, silent consonants, and a structure that seems to string words together seamlessly in a lyrical, almost sensuous cadence. One particularly amorous gentleman described High Mordentish to me as "the only language in which love can truly be expressed." His subsequent attempt to demonstrate this point left me unconvinced, however.
Low Mordentish has a more forceful tone than the High dialect and uses more harsh consonants. Extensive use is made of compound words – for example, a "child murderer" is simply child (beam) plus murderer (myrthra), giving "bearri-myrthra."
In Dementlieu, the High dialect is predominantly in common usage. Virtually all of the commoners also speak High Mordentish, particularly if they work in or near any of the areas frequented by nobles. In their homes, however, they are more likely to revert to Low Mordentish. All forms of artistic expression in Dementlieu are performed in High Mordentish at the command of the Council of Brilliance.
Mordent's use of Mordentish:
I understand that originally there were two similar yet subtly different dialects of Mordentish, one spoken by the nobility and another, coarser version spoken by the common folk. While the two dialects have since blended into one tongue owing to the depredations of time and the disappearance of the noble class, a good number of phrases and even some isolated pockets of the dialects yet remain, especially in the more remote reaches of the region. This experience can come as a surprise to the traveler who, like myself, thinks herself fluent in the local tongue, only to occasionally find large gaps in her understanding even when she is certain the person to whom she is speaking is still addressing her in the same language! Naturally, I have done my best to catalogue a number of these words and phrases in an effort to prepare others who might face the same difficulty; a primer can be found later on in this account.
As for the tongue itself, it is a deep, rather guttural language whose dichotomous origins are clearly evident when one listens to it. The language is a curious mixture of shorthand expressions and rather standoffish utterances, which are used liberally throughout everyday life by even the most refined aristocrats, balanced against a tendency to describe certain things at great length, often with very descriptive and elaborate metaphors. Common people or those long accustomed to each other's company can have whole conversations with nothing more than a brief exchange of rapid, almost unintelligible short phrases.
On the other hand, Mordentish folk meeting each other in a formal environment or who wish to display great respect to another can summon up concise but eloquent language to rival any orator. The effect can be quite jarring, especially if one has only known a Mordentish acquaintance in one capacity or the other. To hear a refined aristocratic colleague converse in the hearty banter of the innkeeper or a hired maid address one as politely as any society matron can be a disconcerting experience.
And for completeness' sake, what the Ravenloft Campaign Setting book says:
Mordentish: This language is divided into two distinct dialects with markedly different vocabularies. "High" Mordentish is preferred by the upper classes, while speaking the "Low" dialect marks one as a commoner. Scholars posit that this linguistic cleft may have arisen from an imperfect melding of two separate, now-forgotten languages. The result is a highly flexible language that has spread throughout the northwestern Core. Due to its association with the culturally advanced domains of the northwest, Mordentish is widely considered the language of literature.
