I am writing an internship report and I wonder if I may use the adjective buggy. I mean, if a computer program has lot of bugs, is it correct to say that the program is buggy?
Yes, buggy is an adjective in common use in that sense, and Thomas Edison talked about bugs in as bugbears or "little faults and difficulties" in a letter almost 150 years ago in 1878.
It is more a conversational term, and would be used as a qualitative descriptor. Often reports look for greater precision, and quantitative descriptions, such as rates.
Code defects is a more formal term, and "defects per 1,000 non-comment lines of source code" is a relatively common way of quantifying the bugginess of the software.