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A more popular language will have a larger community and more community support, which results in more third party libraries or modules and more programmers to hire.
Larger number of people knowing programming language does not mean better community. For example, compare PHP and Ruby (this applies to good quality libraries too).
More than one language can be used in a project.
This also opens the possibility for multiple implementations of the same project.
Popularity increases the gene pool of possible contributors.
In open-source projects, you'll get more contributions if project is in less painful language. Most popular languages are quite painful.
Popularity means it has been proven work.
New languages often become suddenly popular before they're proven in the real world, only for people to switch back again.
Popularity can also arise when there is no other viable language exists for given purpose.
It's not necessary should be the most popular. Popularity just should cross some threshold to signal that it works.
no. It doesn't.