It is expected that in main rooms and national rooms (accessible through the room list under "Rooms" menu), the language in use will be the country's or the geographic area's language: English in the English Game Room, Greek in the Greek Room, French in the Salle Française (the default main room for those with a French client), and so on.
Of course, this rule is not to be enforced in too strict a fashion, and users that try to obtain an information in a main or national room without being able to use this room's language have every right to be welcomed.
The use of different languages in rooms other than the main and national rooms is not subject to any specific regulation.
Rule of thumb: even if it has been created in a room (main or national), a game is neither that room nor an extension of that room. Therefore, the linguistic rules of a room do not apply to the games held in that room.
Those who kibitz a game are free to use any language they wish to use. Whatever your preferred language is, you have no right to try to enforce its usage by others in a kibitz. Tolerance is the rule here.
During a special event (such as a championship game relayed on KGS), an admin may ask to stick to a particular language in the kibitz.