Should I finish one language's lessons before starting to learn another language?
Should I finish up the lessons for the first language I'm studying (French) before really getting into my second language (Welsh)?
It all depends on how much you can handle and if you want to know the language fluently or not. For me, I couldn't juggle two languages. If I did I'd not learn the languages fluently, but everyone is different and it depends on how much perseverance you have, if you want to learn the language well enough, and if you can handle learning 2 languages at once. Good luck on whatever you end up doing! You can do anything you set your mind to!
French and Welsh are pretty different from one another, so this might work (you won't get the words confused in your head all the time). Nevertheless, I think I wouldn't learn two languages at the same time. I would either finish first tree and then start the next, or put the first language on hiatus.
I am actually juggling three languages at the moment (French, Dutch and Spanish), but I had already finished the first two before starting Spanish, and I work on a "one week one language"-basis (two weeks of Spanish, one week of strengthening one of the other two). Works quite well for me so far.
Highly personal. I would suggest that you give it a try. Start Welsh and see if you feel it's too much, whether you start making more mistakes in French (if you want to be very thorough, record how many mistakes you make in French for a few weeks, then start Welsh and you'll have a clear number to compare to) or whether you mix them up. If any of those is the case, stop again and progress further in French, before taking up Welsh. You won't "ruin" your French through a bit of language mixing for a few weeks. If you decide to stick with both, try and separate them in time (practise one in the morning and one in the evening or something similar).
Whoa those are some good tips, thank you! I might try that, studying one language in the morning and one in the evening.
For me personally, I've generally found it more effective to finish a tree before moving on to the next one. This is probably especially true if you are a beginner in both languages. This guy speaks ten languages and here is his opinion: http://www.thepolyglotdream.com/learning-more-than-one-language-at-the-same-time/
It depends how comfortable you feel practicing both languages simultaneously. I personally tried to learn Spanish and French at the same time and it did not work out for me. Since I am more fluent in Spanish than French, I started pronouncing French words with an Spanish accent instead of pronouncing the words properly. I had to learn Spanish first, then when I became fluent in Spanish I started learning French.
In your case, French and Welsh are different languages. I don't think you will have the same problems I did since I was learning two romance languages at the same time. It depends on how you feel. If you want to become very fluent I think learning one language at a time is the best solutions. Well, at least if was for me.