> That assumes it’s harder to build it in the other language though. Maybe if that language is C then that will the case, but building in say Go may be just as easy as building in JavaScript (or close enough that it doesn’t really matter)

The cases where Go is significantly faster than JavaScript are vanishingly small.

> whereas rewriting it later would be a massive undertaking.

This is vastly overstated IME. Porting existing code as-is between languages is actually pretty easy.

> This I very different to say starting a with a micro service architecture which imposes relatively high overheads,

Disagree. You're imposing a huge overhead on hiring (if you want people with both languages) or on people's ability to work on the whole system (if you're happy hiring people who only cover one side or the other). Debugging also gets a lot harder. There's essentially twice as many tools to learn.