That sounds reasonable in theory, but doesn't work out in practice.
In practice we already have our hands full learning the domain unless it's a completely greenfield project. Working out the language and its frameworks makes it exponentially harder when coming into a project.
I've seen it work in practice plenty of times. It does take a particular sort of dev, though.
I don't think that's really that big of a deal. Any above-average developer should be able to become useful in a new language in a pretty short period of time. They don't have to know the language on day 1, they only have to know how to develop software.