Should I learn Czech here on Duolingo if I want to learn Slovak?
You can start for example here and continue with a normal course.
As a Czech native, I'd recommend going right for Slovak. They are very similar but still distinct. It is normal to understand the other, once you reach a good level in one. But the same is not true about the active skills, those could be harder too keep separate. After all, we usually have bilingual conversations among the natives too, each speaking their own language. If you want to actively speak one of the two, learn it directly, don't confuse your beginning phase of learning with the other.
Czech and Slovak are very similar so if you only intend to learn Slovak, I would start learning f.e. on Memrise: https://www.memrise.com/courses/english/?q=slovak. Otherwise you might get confused. But that's just my opinion. :-) However, if you have already some background in another Slavic language you could try learning both. :-)
I wouldn't recommend it if you just want to learn Slovak. Of course the languages are close, but the differences are many and when we (Czechs) communicate with Slovak there is a lot of we actually have to learn first, typically in our childhood by being exposed to some Slovak.
If you just want to learn Slovakian then it probably doesn't make sense as Czech and Slovakian run very close, but also differ quite a bit. Having said that, it is probably much easier for a Czech to understand a Slovak than it is for a Pole to understand a Czech, but it is also quite easy to understand a Slovak being a Pole, so if you learn both Czech and Slovakian or Polish, you will be able to communicate with three nations.
I guess it depends on what's your goal.
Even though everybody is telling you that it will confuse you, I can tell you that in my case going through Duolingo's Czech "tree" actually helps me with the vocabulary and with some useful phrases quite a bit.
Also consider that: 1- If you speak Czech, Slovak speakers will understand you. 2- If you visit Slovakia, you will find that many times Czech is used for the dubbing at the movie theater or on TV and also in the labels of many products. 3- Certain dialects in Slovakia use Czech words.
Now, do have in mind that these are different languages, so if your goal is to communicate with Slovaks, learning Czech will do. If your goal is becoming fluent in Slovak, then getting to know a bit of Czech will help as most learning resources are to learn Czech and very few for Slovak, something that I personally find odd as to me, learning Slovak will help you understanding other slavic languages much easier than learning Czech, but I guess there is more demand for the Czech language.
www.slovake.eu is good, but there is also www.e-slovak.sk, which is maintained by the Comenius University in Bratislava. Also, you could order online some books: - Slovak for You: thanks to this one I was able to skip A1 and went directly for A2 (ISBN: 80-8046-324-7) - Krizom Krazom: There are several levels, A1,A2,B1 and B2 plus the exercise books. - Hovorme spolu po slovensky: Same idea as Krizom Krazom, different authors.
And then there is also Slovencina pre cudzincov, but I didn't like it much.
Also, Lingea has a bunch of books with phrases that will help you with the vocabulary (also available as eBooks).
If you ever plan on living in the Czech Republic then I'd recommend learning Czech. I've been married to a Slovak for 16 years. Lived in America with her for 13 of those. I learned Slovak casually during those first 13 years and I never quite got fluent, but had a quite good grasp of it and could do well in casual conversation. Had I known that I would move to the Czech Republic at some point in my life and had I known what I know now about the differences between the languages, I would have opted to learn Czech instead. Learning Czech with all of the Slovak that I know has been very difficult. Indeed the languages are very close, but the combination of different words, different pronunciations, and slightly different rules/conjugations, for someone who knows Slovak well but not fluently, creates for a very difficult learning process. I imagine if I had been fluent in Slovak before trying to learn Czech then maybe it wouldn't have been such and issue. When I first came to Prague I realized quickly that I had a one-way communication channel. The Czechs understood me most of the time but I didn't understand them. The combination of the things I mentioned above mixed with the accent and my personal feeling that Czechs (at least in Prague) speak faster than the average Slovak was a huge stumbling block. I would understand probably 75% of normal conversation, if it was written down on a piece of paper. The accent, pronunciation and speed of the conversation is a real challenge. It's getting much better, but it's taking time.
I actually do think that the Czech language is easier than Slovak though. There is also a much higher quality of learning resources for Czech compared to Slovak in my opinion. Wish I would have started learning a long time ago.
As someone who learned Czech and lived in Prague, I can kinda understand Slovak conversations but most of them result in my simply standing there stunned thinking, "wow my Czech really sucks, what is this person trying to say?" So I'd definitely recommend just going straight for Slovak if it's the one you want to speak.