If people want to migrate to a specific country long term, the spoken language has to be learned to become a member of society and prevent the forming of parallel societies.

Two points:

• There’s learning a language to a basic level to be functional in every day activity and then there’s learning it well/fluently. Reality is that first generation migrants rarely learn the native language well and it isn’t until the second/third generations that the native language becomes a first language amongst the migrants’ families.
• Given the above, your hard statement about “parallel societies” being inevitable without sufficient language education is false over a long enough time period (~25yrs), as children of migrants will inevitably learn their country’s language and culture … because that’s how children and language and culture work.

All up, presuming that you’re German, it feels like you and your culture might not know how immigration works. Which I say not just to be argumentative but because the one thing that is likely to prevent the above is an entrenched anti-immigration culture that forces the migrants to feel alienated and form more insular cultures rather than integrate with an accepting culture.

Reality is that migration seems to have worked plenty well in many other places. Europe may just be bad at this. And while there may be something to the issue of “protecting the culture” … I’m just not convinced the finer details of any culture are worth protecting at the expense of being shit to others and conservative about how things have to be.