Characters can choose from having one language at +8, or have two languages at +6 and +4.
Example: Milton is an corporate administrator, he had a straight college education and speaks excellent english (+8). Kat went to the school of hard knocks instead; she speaks english at +6 and gutter +4 (gutter being the european streetslang).
You might want your character to know another language. If so the skill costs you only half the character points it would do normally (round all fractions up).
The proliferation of the western media and the Net have made english the common language. That is not to say that other languages are dying out. Spanish and German are very common, the latter often heard in LEO. French is still going strong along with Chinese and Urdu.
There is a big difference between the language taught to us and our common parlance. Using slang, like gutter (european) or jive (american), will allow you to communicate at a basic level, but is not as good as an official language. Most video broadcasts are in main stream languages; ie: english, spanish or german.
Street talk varies from sprawl to sprawl and changes frequently, with words coming and going like all fashions. In areas with a strong corporate presence, you are unlikely to hear street talk, as there are not the people there to speak it.
As a rule of thumb you can break language skills down into four groups: basic, good, natural and fluent.