Galactic Basic (or Old Galactic Standard, or just Basic) was the most prevalent language in the galaxy.
Following the creation of the Old Galactic Republic, the Core Worlds brought their language to all developing worlds, mainly through trade, which was quickly chosen as the new official language of government and commerce. By the end of the Old Republic, the language was known on millions of planets, and spoken by billions of different species.
During the Clone Wars, many Confederate planets and their allies adopted a more primitive, yet understandable version of Galactic Basic which was stored away on Coruscant. The clones fighting for the Confederacy also knew the language, and it was believed the knowledge was implanted into them. Many non-human Confederate leaders and commanders secretly referred to Galactic Basic as "slime", preferring to use their own language.
Under the Galactic Empire, a new version of the language was founded, known as Imperial Basic, spoken mainly by higher up Imperial officers including Wilhuff Tarkin, and even by Emperor Palpatine himself.
Most sentient species that made galactic contact could and did speak Basic in addition to whatever native or regional language they might have used on a daily basis. Some beings normally did not possess the organs necessary to speak Basic such as Wookiees. Whether or not a being could speak Basic, most were able to still understand it