Start from the Minimum-Viable-Product and then iterate from there. If there are pet pieces that the developers would like to make along the way then you let your customer handle prioritizing that work.
There is another side of this issue that you mentioned. If people are trying to push projects into a new language then they obviously have interest in learning something new and are not given an outlet for that energy. Letting them get some down time to work on a small pet project to learn a new language and report back as to how it performs would be great. Help them to understand that there is a time for experimenting and the Customer's product should not be that.
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Start from the Minimum-Viable-Product and then iterate from there. If there are pet pieces that the developers would like to make along the way then you let your customer handle prioritizing that work.
There is another side of this issue that you mentioned. If people are trying to push projects into a new language then they obviously have interest in learning something new and are not given an outlet for that energy. Letting them get some down time to work on a small pet project to learn a new language and report back as to how it performs would be great. Help them to understand that there is a time for experimenting and the Customer's product should not be that.