The Gnomish language is rapid-fire and atonal. With almost 200 phonemes (modern American English has 57), a fluid grammatical structure, contextual vocabulary and the Gnomish love of metaphors, idioms and wordplay, it is considered by scholars to be one of the most difficult languages to learn in the entire world (Elven is still harder to speak, though).
Sounds like Welsh being spoken with a Yiddish accent.
The written form of Gnomish is equally complex with 137 distinct characters that are contextually-phonetic.
Pragmatic Gnome engineers have developed a dialect of Gnomish, known as Gnomic, which stripped out the idiom and standardized the grammar, but kept the multi-layered structure. This resulted in a very efficient language for the purposes of technical discussions and research, and to a lesser extent alchemy, arcane magic and combat. However it is limited, and has trouble with abstract ideas and expressing emotions or feelings.1
1 -10 penalty to Perform checks for any use of the skill involving speaking in Gnomic.