Can property outside the EU be counted as equity?

Thank you for the answers, they confirm my assumption.

I meant it as "security" for the bank, I will be more careful with the choice of words next time. Then apartment 2 will probably have to be sold in the medium term and the concrete gold will become equity.

Is there even a land registry office in China where mortgages can be registered and then possibly foreclosure can take place if payments are not made? If at all, a large international bank takes the apartments as collateral. However, effort and costs are likely to be high. What amount are we talking about for the new apartment and how high and secure is the income? If it is a new job or a different company, you can forget about it anyway. Do employer loans still exist?

I hardly believe that a German bank would be willing to enforce a foreclosure in China :D. No, these properties do not serve as collateral at all.

Rental income could certainly be counted as income by some banks if you can prove that it is regular and secure. But currently, you have no rental income: you live in one apartment yourself and the other apartment is vacant.

Before you sell, you can first inquire about the conditions with low equity. You can also, as you write, speculate (not meant negatively) that the added value of apartment 2 in a few years will outweigh the bank's worse interest conditions.

Is there even a land registry office in China where mortgages can be registered and then possibly foreclosure in case of non-payment?

If you buy the apartment and need a bank loan, this is recorded at the land registry office.
This prevents you from selling the apartment without the bank's consent.
Once everything is paid off, you have the bank confirm it, you run to the land registry office with the document, and they remove the entry.
Moving within the company, but the tendency seems to be: contract limited to 1 year.
The boss would have liked to make it permanent right away, but the HR department said "we don't know him well enough" – even though I've been working here for quite a few years. So the question might only be hypothetical and the bank says "Come back in spring 20222."

Confirming rental income will certainly not be easy. Especially since 1-year contracts are common in China that are renegotiated every year aka "the landlord asks for 50 euros more every year."

Conditions without selling apartment 2 probably won't be great because then I would basically need 100% financing, my savings only suffice for a car and apartment furnishings. But asking doesn't cost anything.
In case of a real estate purchase, it will probably come down to selling apartment 2 and using the rental income from apartment 1 for an annual special repayment. That's not so bad after all.

Just have it calculated what a sale would bring you. Whether you get much better conditions through the cash portion. Otherwise, rental income will be credited to you if you can prove it, preferably taxed. Rental income from abroad will probably be rather difficult, I think.

How does your income look? Abroad or Germany. That will probably be the crucial point.