The most common language in the Pocket Universe, called simply Common, is mostly derived from English, the most common language spoken by Humans on Earth in the Old Universe. Common also includes some words derived from the language of Shi'ar and even loan words from Orks and Sontarans.
Common emerged as the lingua franca of the Pocket Universe in the Early New Shi'ar Imperial Era, and was very frequently used by most inhabitants of the local universe by the end of the Era of Anti-Imperial Rebellions.
There are certain exceptions. The speech used by Space Gypsies and Mandalorians is more purely English and has far less use of Shi'ar, Ork, and Sontaran words. The United Pirate Clans tend to speak in pure Shi'ar speech. The Star Elves and Asguardian Dwarfs still mostly used their own racial tongues.
The language of Common has in recent centuries and millennia has split into certain dialects. The High Common dialect of the Common language is frequently used in the New Shi'ar Imperium, by members of the Trade Federation, and in many parts of the Systems Commonwealth, the Alpha Quadrant, the Beta Quadrant, and the Gamma Quadrant. The other major dialect of the Common language is Delta Quadrant Common, which is very commonly used in the Delta Quadrant, uses slightly fewer Ork and Sontaran words and is influenced by the British accent in the Interplanetary Neo-British Kingdom and the use of religious terms in the Kurgan Caliphate.