I use the Accept-Language per site add-on [1] for this.
It allows me to map top-level domains to preferred languages. Most of the time, this has worked well for me on sites which respect the header.
For example, `.de` sites tend to be originally written in German, so I map the `*.de` pattern to the German language.
No, it is not enough, because there is no way to tell: "Pick the source in my 3rd language rather than the translation in my 1st language".