I want to learn a difficult language (Japanese) and an easy language (Spanish). Should I learn them at the same time?
Stop sucking at two languages and get good at one.
— from the AJATT site.
It is possible if you manage it, but I don't recommend it. Learn the easy one first and do laddering and/or find a way to maintain Spanish while learning Japanese so that you don't forget it.
I started Japanese after I was already proficient in English. I learned one then went to the other. So even though I still regularly immerse in English, I can dedicate almost all my time to Japanese. If I started Japanese and English at the same time, I would definitely fail at both.
All European languages are closely connected. They are similar to each other. They borrow large chunks of vocabulary from each other. So learning another European language will be easy. It should take you around a year to basic fluency. But if you do it alongside Japanese, you may never see the end. Japanese is so deep and complicated that even after reaching fluency you're going to have to polish your skills and learn some corner-case things here and there. Due to the above, starting a new language while doing Japanese is nearly impossible due to the effort required to maintain and improve at Japanese. If you simply drop Japanese, you're going to forget a large portion of it during the time of learning another language. So I recommend doing the European language first, no doubt about it.