Here is my American English correction: In addition to the language courses, they were completely helpful, quick to answer all my questions, and gave me detailed information about everything. They also gave me a very useful map of the city's transportation system, and accompanied me during my first days there.

It is easier to answer if you post such questions as a notebook entry. The first phrase needs some more explanation: for example "In addition to the language course that they [[who?] provided, they were also .... " to answer to all my questions --> to answer all my questions informations --> information transportations --> public transport The comma before "they accompanied" would better as a semicolon. "completely" is the wrong word to use before "helpful": I would omit "completely" but you could use "very".

In addition to the language courses, they were completely helpful and quick to answer all my questions and gave me detailed information about everything (they gave me a very useful map of the city's transportations, and they accompanied me during my first days, etc.)