Yes, Spanish and French are easier to learn if you know one of them, but, at the same time, many words and things are confusing. There are lots of "Faux-amis", words that appear to be translations, but they are not. You have things like "livre" which means book in French, but in Spanish you have "libro" (book) and "libre" (free), so if you don't pay attention you write "livre" or "livro" in Spanish.
Also, things like the word for blood. "Le sang" is masculine in French and "La sangre" is feminine in Spanish.
About Japanese, the words are not similar, it is just that they are pronounced similarly. Also, I think Japanese have a good relationship with Germany, so many words were adapted from that language. The same happens with English, as when Japanese don't have a translation for an English word they invent a word that sounds similar. I can't think any example now, but I swear, there are many. You realize that when you see animé in Japanese with English subtitles.
I always try to see things in the original language, with subtitles. And with Japanese, many times the Spanish subtitles are not good, so I prefer English subtitles.
And you are right, excellent training for the brain!
Oh. I am doing an online course about satellite images. I had some idea for previous courses but I had forgotten most of it. I am having fun too.