The language is Bole. Kapa ga nossa = Until later / Until with passing-time.
The response plays on the meanings of nossa, which can mean 'to spend time' or 'to rest'. The phrase, "Kapa ga nossa!" can best be understood with the meaning "passing time", but the answer can then use this word in the meanng "rest".
A similar play is heard in the equivalent Hausa phrase, "Sai an juma!", literally, "Until one passed-time." The verb juma can mean either 'pass time' or 'last a long time'. A response to the Hausa phrase is thus, "Mu juma da yawa!", using the second meaning, i.e. "May we last a long time!, May we live long!"
BOLE is a language spoken in Yobe and Gombe States of northeastern Nigeria. About 250-300,000 people speak BOLE, making it among the largest languages in the region, perhaps surpassed only by its linguistic cousin HAUSA, the dominant language of all northern Nigeria and the sub-Saharan language with the most native speakers, and KANURI, the historically dominant language of northeastern Nigeria over the past few centuries. BOLE is a member of the Chadic Language family, and belongs to the same main sub-branch of that family as Hausa, though linguistically Bole and Hausa are probably no more closely related than, say, English and French.