The more time I spend in the industry, the more I tend to believe you should just hire programmers who have a demonstrated ability to produce a high-quality product. Any product. Programming languages and domain-specific knowledge can be learned quickly, and all the best practices and process controls in the world can't replace talent and experience solving problems.

Those things can sometimes be learned quickly, but it takes time. If your company can't have, or doesn't want, productive developers to be spending a lot of time mentoring, then it can be worth it to hire people trained in your working language, possibly even familiar with your style of program or library ecosystem (if you have one of those).