What you want IINA to do:

I want IINA to use speed markings in values such as 1.2 and 1.5 (as opposite to the current 1,2 and 1,5)

What IINA does currently:

The UI uses . for the marking beneath the settings (see 0.25x)

Why you think this should be added:

I believe it used to be default behaviour of IINA, until on some version I began getting the error bleep when I wrote the more internationally accepted . delimiter within a value. I only recently understood that it wasn't a bug, but a behaviour, and that I had to write 1,2 instead of 1.2. It is my understanding that Americans favor the comma, but just about every app out there that allows changing the speed shows the value using a period (YouTube app, Podcasts on iOS, etc).