I had the same problem and solved in this thread: Wrong locale pt-BR - #4 by Petter
You can add lang=pt-br to the URL to change the UI language on the fly. The docker image of collabora CODE already come with all language packages installed. If you are not using the docker image you probably will have to install your specific language package: Installation from packages — SDK https://sdk.collaboraonline.com/ documentation
If there is another way to change the UI language without relying on the URL parameter I would like to know.