- What third party libraries do you need to use? Some languages have very good support for some, and less for others.
- What are the internal integrations you need to support? Can they be over the network or are you calling into code in a particular language?
- What is the pool of skills available to you as a team? Do you go with a language that has a reputation of being really good for this task but of which the team knows very little (and therefore will have a learning curve working out the common pitfalls), or do you go with a better understood language which the team has already mastered, and stretch it to go beyond what mere mortals do with it? Note: there's no right answer here, both options have severe drawbacks.
- Related to the previous: what's your company's culture regarding technical diversity?