The language agents write in
Set it once for the whole workspace.
In Settings you pick the language your agents write in.
Match the person is the default: each agent answers in whatever language you wrote to it.
Pick a specific language and every agent writes in it, even when someone writes to them in another one. That is useful when the team is mixed but the output - summaries, posts, reports - has to be in one language.
A single agent can still be told otherwise in the chat; a standing rule on that agent beats the workspace setting.