[rplnt]

This makes it hard to master languages similar to your native one.

With a completely foreign language you have to pause and think. With language close to one you already know it's easy to fill in the gaps by switching the language.

[kmmlng]

You'll also have a hard time really internalizing vocabulary. If you're learning Dutch and you already speak English and German, you can quickly get to a level where you are able to read Dutch texts. This is because half of all Dutch words have very similar German or English counterparts. But since you just understand them effortlessly, your brain doesn't actually learn them. You'll find yourself trying to say something in a conversation and the words just won't come to you, even though you would have no problem understanding them in written text.

If that happens you can always flip a coin and just either dutchify the English or the German translation of what you want to say, but it's not the most elegant solution.