Sorry if you felt talked down. I am lucky to have worked in some very big and complex React applications and will share more experience with you:

* By using a singleton you're not able to easily test different LocaleData contexts in parallel.

* By using a singleton you're not able to make your NodeJS server deal with users with different locale settings because you have only one global instance. Multiple requests arriving at the same time would corrupt each other. You could spawn a separate process for each locale but that's inconvenient.

Does that make sense?