GT doen’t seem to work with Observable notebooks, actually only comments are translated. I don’t know if there is much to do about this. Maybe other translation tools or extensions, which i am not aware of, could help?
I’d find useful for people who’d like to write their notebooks in their own language (german, italian, russian, whatever) to be able to do so, and get easily read by others as well. After all, notebooks in german or french would confirm that Observable can be used in the real world, and for the general public, everywhere!