This is certainly a very nice feature. A GUI would be needed in the settings manager so that the target languages could be set better. For example, I from Germany with German as my native language would like two languages: - English - German
So the simplest solution is certainly to set a primary language in the Settings Manager.
This should be freely adjustable because, for example, the Firefox beta is only US and you would get an incorrect entry.
It will be very useful to have the possibility to use the right click on a website page to have translate the page in your language preference. Like you in the other navigator chrome, edge...ect . Because now you need to go on the adress bar click on the translate icon and click on translate the page but it will be faster if you just have the right click like also other translator extension in firefox. It will be very usefull to have that for the official firefox translator.