Is the purpose of the project to learn a specific new language or framework? Problem solved. Does the project involve certain functionality that's already been

Is the purpose of the project to learn a specific new language or framework? Problem solved. Does the project involve certain functionality that's already been solved in a framework or library available only in one language (or at least not in any languages that my team and I are already proficient in)? Will it be faster for me or my team to learn that language thoroughly than to reimplement the functionality? Is the project being implemented on a platform that only supports a limited set of languages officially (e.g. iOS or client side web development)? Pick the most widely used of those. Otherwise the project gets done in the language I enjoy most and am most proficient in (for personal projects). Because that'll be the most fun for me. Or the language my team collectively knows best and were hired for their knowledge of. Because that makes knowledge transfer and future maintenance easier.