It is hard to confuse English and Math as they are dissimilar. However, the differences between styles may be small. A closed fist here versus an open hand there. Upright stance in one, lower stance in another. When I was in high scool I took classes in English and Spanish. I became so fond of some Spanish nouns that people caught me using them when I was speaking English. And the verb "es" (pronounced "ace" in english) which means "is" in spanish became part of my regular vocabulary for several years. LOL
Language is actually a pretty good metaphor. I'm going to continue with that for my explanation - for the below I'm making an assumption that all the people referenced have roughly the same intelligence level and aptitude.
The first important thing is the age that you started. If we took you (if i remember right you're in your 70s?), me (30s) and a teenager, and had all of us really devote ourselves learning japanese, it would probably take you 5-6 years to become fluent. At my age, it'd probably take me about half that, 2-3 years. A teenager is probably getting fluent in a year. The younger you are, the easier it is to pick things up, adapt, and remember things, ehich holds true for martial arts as well.
The second important thing is that similarities actually help. If I'm learning italian but already learned spanish, I already have a basis for sentence structure, understand a good number of verbs that are similar, and know what methods work/don't work for me to learn further. If my friend starts italian at the same time as me, but never learned a language before its going to be much tougher for him even if I do occasionally accidentally use spanish words and he doesn't. The same holds true for martial arts - the person who already understands the mechanics of striking, memorizing forms, doing drills, and timing will have a much easier time picking up a second striking art then the person who knows none of that. And in MA there's an added benefit that including things from your other martial art occasionally actually helps you, not hinders you.
The last thing is that for languages that are different, you don't have to worry about any mixing them up, but the method you used to learn still sticks around. So if I'm a native english speaker, learned spanish and want to learn japanese, none of my spanish will help me, but I already know how to think in another language, and I know what stuff works/doesn't work for me (ie: personally flashcards help a tiny bit but really I need to read sentences and work backwards to really understand and piece them together, reading paragraphs grammar rules also helps me a lot even though for others it might just confuse them). Which holds true if I know karate but want to learn judo. Karate might not help me directly, but I already have a pathway to learn I can follow.
This is all ignoring that some people genuinely do have more aptitude then others just naturally.
When you combine all that, it explains why there are so many polyglots knowing 5+ languages fluently, while other people struggle learning their second language. And why the same is true for martial arts.
