I started learning Welsh some three years ago, mostly with SSiW, and have finished gogledd 1/2/3, level 1 and now I switch between level 2 and the DuoLingo course, since two weeks ago when it came out.
A year ago, I started some Irish on DuoLingo too, just to check it out. Later though I decided to start Swedish (just out of curiosity) and then also Italian. I have had half a year of Italian in school, 35 years ago, but never was far enough to understand or even speak it. I always worried about the effort spent without return, and because some Italian is always around us here in the Swiss-German speaking parts of Switzerland (e.g. most products are also labelled in Italian, because of the Italian speaking areas, and there are plenty of Italian speakers to use as practicing material), I decided to push it to the point where I would passively understand 100%, and have simple conversations too).
Time however. Irish is rusting quickly in the back yard, and I only do one hour of Swedish a week, to keep it from evaporating too quickly. I hope to save some Swedish over the Italian job, which will not take too long - with my head start, I should be able to finish it this year. I am still working on Welsh, and interestingly, it does quite well even after several weeks of neglect!
I had French and Latin in school, too, but both with window seats. The remaining fragments do help with Italian, though. I also consider doing a Cambridge proficiency exam (tests say I would just about pass, but I would want some space to feel safe, or is it room, you see what I mean). I found that there is hardly any confusion between English, Swedish and German, every little confusion or false friend is 100 times amortized by the profit from common stock. Likewise, neither did the pair French / Italian caused problems, nor did intensive learning of Irish and Welsh at the same time cause much trouble.
The only conflicts are for time and neurons.
In fact, I learned that in Italian both “ti amo” (unstressed you) and “amo te” (stressed) work, but “te amo” is Spanish - they explain this to bust confusion, I guess.
I do however, not do more than three languages per day, and do something in between, so the smoke clears.
But I do think that some confusion would happen if I did several languages too closely, in sub-concious processes.
Iechida!
Martin