If question is regarding SIP/VoIP .. then negotiation between both the ends is based on offer-answer model defined in the SIP.
let's say A party know 3 languages ( English, Hindi, Kannada) and B party knows only English, but in SDP sent from A to B will have all the capabilities included but in Answer for this SDP contains only supported languages, it means B party is agreed only to speak in English.
Note: I just given example of languages for understanding purpose, in SDP we will send codecs i.e. voice/video..etc ..
