Is it permissible, Islamically, to deal with non-Islamic European banks and companies that deposit money and give 30% monthly profits for one year, considering the principal as part of the profits at the end of the contract?

The aforementioned transaction is forbidden interest (riba) because the reality of the deposit in these banks, with the guarantee of capital and profits, is a loan with interest. This is clear usury (riba), and it is not permissible to engage in this forbidden transaction, whether in the land of disbelief or Islam.