Recommended Language/Framework for Building a New Recommendation Engine
I am developing a recommendation engine for stack overflow (personal project). Check it on http://recommender.im . 

It is still a working in progress, but I have a quite functional website working. I am putting there most of the code I used through python notebooks.

Basically I used:
- Frontend: angularJS
- Website backend: flask + scikit-learn
- machine learning and data preparation: python, pandas, scikit-learn

I really like python for data science as the community and libraries are really good.