I have a browser whose default language is Spanish. In chatgpt chat interface (in Spanish), I write instructions in French that requires searching information on the internet and reasoning. During the thinking phase, ChatGPT 5.2 Thinking generates reasoning summaries in Spanish. However, when the reasoning ends, its final response is all in French. Does this mean there is a way to control the language in which reasoning summaries are emitted? Can one do the same with the Responses API ?
I have a system based on the responses API where the system message, the user message and the generated output are all in French. However, the reasoning summaries are still displayed in English.