As an early engineer at a startup, you get to make the choice of what language to use. This is a big decision...choose TypeScript.
Before I go on further, there are some edge cases where this is not the case.
You (or your team) have deep expertise in another language (i.e. Ruby, Go, etc.)
You (or your team) are building specific technology with a dominant language (i.e. Python for machine learning)
You (or your team) want to optimize for fun/happiness and another language provides that
Now that we got the edge cases out of the way, here is why (most) startups should only write TypeScript.
There are huge benefits to using one language for everything. We can share code and every engineer can be full stack.
In the early stages of the startup, it's all about finding product market fit as quick and efficiently as possible. We can choose to optimize each product if necessary (i.e. converting React Native to Swift) once the startup is mature.
Advocates of obscure languages will claim choosing such a language will attract better engineers to the team. This may be true in the beginning, but costs soon outweigh the benefits as the startup scales.
Recruiting and onboarding engineers is hard enough, we shouldn't make it harder by choosing an obscure language. Everyone already knows TypeScript or can pick it up within a week because of its familiar syntax and robust community. There's also endless packages/libraries to speed up development.
TypeScript provides just enough structure to satisfy static type advocates and just enough flexibility for dynamic type advocates. It's the happy medium that will prevent your team from looking for something else.
If you are in a situation to choose a language for a startup, make the pragmatic choice and go with TypeScript. If you go with any other language, the team might end up resenting you in the future.