Russian

— Presumed to have been written in the diary of Oleg, The Destroyer as he lay on his deathbed

This is language of powerful races. Notably spoken by most Goliaths, some Orcs, some Dragonborn, and some species of Genasi.

Phonology

Having originated from the Kievan Rus, a group of tribes living in present day Russia, Russian has five (or six depending on who you talk to) vowel sounds that can change depending on the preceding consonant sound. These pairs of vowels are commonly called hard and soft. Stressed vowels can last a bit longer when spoken.
Consonants are really cool in Russian. They have a more retroflex shape to them in the mouth than normal English consonants.
Combining vowels and consonants is easy. Just smash a bunch of consonants together and forget to put a vowel in there until you've got too many consonants and don't know what do to with them. Don't believe me? Here's a word you can try to pronounce: взгляд (pronounced vzglyad) meaning glance. Try making that first sound. Here's another: государств (pronounced gosudarstv). This time the consonant jumble is on the end of the vowel.
Not good enough? Here's a quick tip! Just smack a bunch of these 'Ь' in the word - it makes it more better. SeeЬ? ItЬ's alЬready loЬoking bЬetter! Tьheьy cьan alьso ьe smьalьl tьoьoььььь. ь.

Morphology

The shape of letters is derived from Slavic languages dating back to the 1400s when the alphabet sort of settled down and became what we know today as the Cyrillic alphabet.