ju … desto is a combination, in English the combination is the … the
These expressions work like this: If you have more X, you will have more Y.
Ju mer du läser, desto mer lär du dig - The more you read, the more you learn
Ju större fötter du har, desto större skor behöver du - The bigger feet you have, the bigger shoes you need
Ju fler desto roligare - The more the merrier
Ju fler kockar, desto sämre soppa - The more cooks, the worse the soup (Proverb: too many cooks spoil the broth)
I thought some more about this and came to the conclusion that ja is the best translation after all. In doch, I feel there's always a sense of contradiction, and in most cases it would be better translated with fast (or fast + ju) in Swedish (although fast works a little differently syntactically). And fast is definitely a very false friend :)
Is there a difference between using "Ju ..., desto ..." and "Ju ..., ju ..."?
There's this Swedish article here: https://sverigesradio.se/sida/artikel.aspx?programid=411&artikel=3240910 I believe it says that using "Ju ..., ju ..." is more colloquial. Am I interpreting this right? Is there anything else worth knowing about this?