I'd say "You can pick up 90% of any language or planform in a few weeks." That last 10% though is some edge-case/deep understanding that you won't even know about until you really need it (and it's probably shown up in production).
I think that different languages have their own level of how much edge-case there is in total, as well as how much may be exposed edge there is visible now vs later. It's akin to having icebergs of different volume, but unlike icebergs, also different proportions of 'ice' below and above the water.
I'd say "You can pick up 90% of any language or planform in a few weeks." That last 10% though is some edge-case/deep understanding that you won't even know about until you really need it (and it's probably shown up in production).