As someone writing an AI library in Typescript - if AI is your main focus, learn Python. You'll have access to the largest AI libraries and frameworks with the most support materials and userbase.
I think it has fundamental DX benefits over python for complex prompt chaining (or I wouldn't be building it!) Even still -- if their focus is purely on AI, python is still the better choice starting from scratch. The python AI ecosystem has many more libraries, stack overflow answers, tutorials, etc available.
- It is already possible to run LLMs and langchain.js directly in the browser. The web is the dev environment if you code JS.