Interesting, I feel like I have the opposite issue. Coming from Portuguese, learning Spanish was a breeze and I can usually surprise French speaking people with “hard” words in my basic French sentences — just because I reached out to a Portuguese word and “frenchyfied” it.

Going from native Portuguese, then French, and only then Spanish (yep, I've gone to a crazy school), I got much of the problem the GP was talking about. I don't think I've ever spoken so much French as the time when I was leaning Spanish.

There's something with those two languages in that they interfere badly.

Spanish and Portuguese seem to be unusually close. Studying one helps me with the other.

Portuguese and Japanese on the other hand...

The OP wasn’t talking about Japanese. They were talking about European languages- many of which share a common heritage

Not all of them do, but the ones mentioned were all very similar Romance languages.

I’m aware not al of them do. This is why I said “many” and not “all”.

There are roughly 3 main groups of European languages: Italic (or Romance as you described it) for Western Europe, Germanic which is predominantly Central Europe, Scandinavian countries and the UK; and Balto-Slavic for Eastern Europe. Generally speaking of course.

However there is still a fair amount of cross pollination even with the Germanic and Italic languages, not to mention shared characteristics (not least of all a shared alphabet) that doesn’t exist with Japonic languages such as, well, Japanese.

I think this dyslexia happens more with non native languages.

Like when I was trying to speak german, after just some time in France. My native language is Portuguese

German and Portuguese differ significantly more than French and Portuguese, so that might be affecting your experience…

I’m not too familiar with the Southern European languages, but I am quite familiar with the Northern European languages. That is where I find the opposite of your difficulty happen, where all those languages look like a different flavour of German to me, so they were very easy to pick up, because I could just read Swedish slowly without ever looking at a textbook or dictionary, and over time I found myself quickly able to write it.

Differences between people, or differences between cultures perhaps. Having grown up in the Netherlands I did already come across Frisian and Limburgs, which are also slightly different but if you keep reading or listening you just pick it up. So I don’t know, keep practicing I’m sure you’ll get Spanish!

It's true that learning one Latin language (like French) makes it easier to understand another latin language (like Spanish), and the same is true of learning similar programming languages. But the point isn't that it's easy to approximate what is being said, it's that when you are speaking, all your prior knowledge confuses you.

For example, if you start speaking (not listening to) Frisian and Limburgs, you might find yourself throwing a few Swedish or German words in the mix.

I have walked up to a shop keeper and asked [french(Can I have a loaf of)][Chinese(bread)]

I'm white so I'm sure this was even more confusing for him

When I started learning Mandarin (my fourth language) I became totally unable to speak French (my third language) as the Mandarin words would pop up first in my mind. I think by now they've been sufficiently separated in my mind but I haven't had any use for my French so it as atrophied anyway. I'm now learning Cantonese which is similar to Mandarin and I'm only occasionally mixing it up with Mandarin. It seems to me the distance between languages isn't so important, you can mix up any two languages or not. It probably depends more on how much your brain has to rewire itself to acconmmodate the new language?