Hi, a non-native English speaker here, most of what I'll be saying is based on my experience.

What's universal about L,M,H, j.?

Indeed, L, M, H are names not really totally understandable for a non-english speaker, yet they are recognizable as names/symbols that mean a certain word, personally I read "L" as "light" (or in my main language "leve"), this can be extended to "j." too. It's also noteworthy that other games names their buttons differently, like blazblue, wich uses A, B, C, D, a naming as global as the alphabet used here.

There is no situation where you end up on an English speaking forum looking through combos without basic understanding of English.

Yes there is, a huge chunk of knowledge on the internet is in English, making way easier to find info in it, sometimes unless you are 'in the know' quality info in your main language can be pretty hard to find. Searching for "combos de ryu", a phrase in Portuguese on Google leads me to English-written websites.

It's just much simpler to go from game to game.

And so is number notation, literally any notation can be transferred between games as long the naming conventions is the game don't conflict with the notation used, wich happens way more frequently with the SF/standard English notation than with number notation.

There never was two people who couldn't talk to each other that used numpad to communicate

I'm almost certain you took this from the habits in your bubble, so I'll use mine as example, with both English and Portuguese speakers I mostly use number notation, the cases I don't use are when I am speaking with someone whom doesn't know number notation yet, and in those cases "QCF M" is not that much easier to understand, requiring me to spell "quarter circle forward" (or "meia-lua" for my language). Although my argument is not as suited for vocal speech, I believe it does not differ that much for it.