Abyssal

Abyssal is the primary language spoken by Tieflings. It has a distinctly long list of vowels and is loosely syllabic, leading some to comparisons with Infernal (which actually doesn't sound at all similar, but structurally is interestingly familiar). It's a lot easier to understand than Infernal however, with tenses and objects and using the Aldyn alphabet, though at one point it had an alphabet of its own and it still adds a few new characters.