When you set a content language in Agent Settings, everything the agent generates — queries, personas, business profile, posts — comes out in that language. Existing content stays as-is; the setting only applies to what gets created from that point forward.
If an agent is running in a non-English language, the Personas and Topics pages show a small pill indicating which language is active. Clicking it takes you straight to Settings, so you're never left wondering why the output looks different.