Common

The most ancient of languages, the one spoken by many species, translated by many, understood by few.
Used by poets, musicians, novelists, orators, lawmakers, kings and other potentates, abused by politicians, it is used by many cultures knowingly and unknowningly. When a culture is introduced to another outside of their current knowledge, many words and terms are grabbed from common, sometimes slightly modified to put a local language spin, and then becomes part of the vocabulary.
While legal documents, novels, treaties, and other important pieces of literature are usually originally created in the local language, especially if that is the native tongue of the creator, anything that can, should, or needs to be shared is usually translated into Common first as many cultures are literate in it. If that is not the case, then it will be retranslated into another local language, however Common remains the bridge and the standard for what was originally written. As such, intergalactic archives consist primarily of documents or files in the Common language rather than any particular local or cultural language. 
Unfortunately, there are forms that definitely lose the ambiance of the language when translated. Music, poetry, and references to obscure local objects such as plants, animals, and especially ancient or extinct objects become difficult if not impossible to translate. In that case, Common may adopt the local language construct and thus, Common is an everchanging dynamic language itself. This can cause grief when two words that mean the same thing are constructed without knowledge of the other and then spreads through the galaxy causing even more dichotomy and confusion within the language.
As such, Common is considered a difficult language to learn and nearly impossible to master. Difficult is not meant in the sense that one needs a high education to learn, as most learn the basics as school children. Difficult as not even the most advanced computers can keep up with how the language changes on a daily basis.