> E.g.: If I want to learn a new language, does the discomfort need to relate to the language, or is it enough to do something like taking cold showers?

Weird to restrict the focus of growth to one particular area, and to ask whether any discomfort (even physical) will contribute to your growth in that particular area, when the abstract just refers to "personal growth" and makes no claim related to some spooky cross-pollination of discomfort. It even gives a list of examples, and refers to feeling "awkward."

But when it comes to learning a language, some discomfort when you're using that language is obviously when improvement in the language is happening. If you're only in situations where your current knowledge is completely adequate, you'll never have any reason to get better. You're not getting better at anything if you're never breaking a sweat while doing it.

edit: could "results showed that these effects occurred only in areas of personal growth that cause immediate discomfort." be any clearer?