IMO framework + codebase. Even if we're talking about companies using off-the-shelf frameworks and not something homemade, the project using that framework uses it in its own, peculiar and usually undocumented way, and figuring that out is even more difficult than figuring out the framework. This cost is present in pretty much every software job.

Absolutely.

It grinds my gears to hear that legend over and over -- "new language is very risky and slow to adopt!", even though a lot of us know that the language itself is the lowest barrier of all.

On top of framework and codebase I'd also add organizational or team coding style. It becomes a bloody mess pretty damn quickly. I've personally spent 6 months being onboarded in a very finely tuned and hacked to oblivion Rails project -- a technology I knew pretty damn well at the the time -- and then it took me 3 weekends to get my own Elixir / Phoenix project off the ground... and for it be useful in business terms as well.

So these "new language risks" are vastly overstated and are simply an excuse for dogmatic people to not act in the project's best interest.