Clozemaster has a Slovak course from English. Taking it as beginner requires some fortitude; might be better to start otherwise. But Clozemaster skills are not completely useless even for a total beginner.
Anki(droid) also has Slovak decks. I have not tried them.
I found wonderful app, named Ling app similar to Duolingo, but there are many functions that are easier to use than Duolingo. For example, this app has a chatbot function. It is excellent. You can practice listening, speaking, writing, and reading by using Ling app solely. And this app can use on mobile and PC. There have more than 60+ languages including ...
Anki is a flashcards app that many people use to learn languages. You can create your own flashcards with words or sentences, or download existing decks.
Tatoeki is an app to browse sentences in many languages and search for sentences containing a specific word. Some sentences have recorded audio. You can create flashcards from any sentence.
I recently found an app for vocabulary named "Vocly" which is you can learn vocabulary with games like word flashcards. It is easy to remember and fun. There have many categories. I really love it. Now, I'm learning Korean vocab. This app is suitable for me, so I want to recommend you, and this app also has Dutch. I hope this can help you.