That is not logically guaranteed. The fastest path to a prototype may in fact involve learning something you're uncomfortable with.
If you're most comfortable with a spoon, the fastest way to dig a trench is still an excavator, even taking into account the time needed to become comfortable with operating one.
If your a pro with C and you really want to build a rest API, then yes using C is a bad choice.
But if you know Ruby, Python or NodeJS you'll be fine with any of the 3.
Then again, newer programing languages tend to cater to what we already know. Flutter/Dart is a dream to anyone who knows Java or Javascript.
Actually, when I think about it I learned Python since I really wanted to get into machine learning a few years ago. I was able to increase my income by around 80k, so it was very much worth learning.
If you're most comfortable with a spoon, the fastest way to dig a trench is still an excavator, even taking into account the time needed to become comfortable with operating one.