Polish sovereignty is threatened whenever a German person buys a chunk of Polish land, for by this he becomes a legal owner of Polish land and can make decisions what to do on this piece of land.
Hardly. You're obviously not very familiar with Polish law, because there exists a very effective mechanism for compulsory purchasing land from owners at the current market rate should it be needed for whatever reason. It's why Poland was able to build a large amount of roads easily, as the actual ownership wasn't a barrier when it came to building.
Let's introduce a law forbidding selling land plots in Poland to foreigners.
Let's restrict purchases by non-EU citizens, yes. But to EU citizens? Rather not - as it just means that EU countries will apply the same restrictions to Poles. More to the point, foreigners are often able to apply their know-how and do more with the land that they have.
That's true that there is too much foreign capital in Poland.
Alas, you can look at WW2 and the PRL for that one. Less foreign capital means less money in Poland.
If someone manages to find more, the fingers of a single hand will be enough to count them.
Solaris, PESA, Tymbark, Black Red White, Empik, Mlekovita - off the top of my head. However, it's worth pointing out that the same situation exists in many EU countries - the UK for instance doesn't have many entirely British-owned companies.
The partially state-owned model isn't entirely a bad one, though.
In Germany or France there is many such companies.
Many of them are actually controlled by foreign capital. For instance, Metro AG (Makro, etc) has around 50% of their shares traded freely in and outside of Germany. It might seem like it's a German company, but in reality, very large institutional investors tend to own large amounts of such companies.
No, it's the fault of Poles, that they are not enterprising enough.
Very much so. Solaris is a fantastic example - they started by producing licenced copies of Neoplan buses, earnt cash, then used it to produce their own buses (and now, trams) which have been a success story. The problem is that there's such a deep distrust of private entrepreneurship in Poland (PiS, we're looking at you) that many people are discouraged before they start. Yet there's such a huge demand for Polish products domestically - but consumers expect Western standards, and many Polish companies fall short.
A friend was negotiating here to buy a considerable amount of mattresses from a factory run by an ex-PZPR guy that bought the factory in the early 1990's. The factory is nearly dead, so you'd think that the managing director would be willing to bend over backwards to secure the order. What happened? My friend turned up for the meeting, only to be told that the guy wasn't there and wouldn't be there that day. Unbelievable, but shows the utter contempt that many Polish-owned businesses have for their clients.
Even on a local level - I've got Biedronka, Kaufland and Piotr i Pawel nearby. Customer service in Biedronka and Kaufland is consistently good. In Piotr i Pawel (which is 100% Polish owned) - they've got a permanent attitude problem. Why would I buy anything there when the woman at the till is chewing gum and treating me as if I'm a nuisance?
People aren't going to buy Polish just because it's Polish, but they will is the product is as good as the foreign one.
In Poland there is many small private stores, sometimes it's also possible to meet a private, fully independent supermarket.
I don't know if it's private, but there's that T&J chain of mid-sized supermarkets in Wrocław that always comes to mind - excellent range of products, decent enough customer service and competitive price wise.