You drill a single hole into an exterior wall and you mount a mini split system. You don't need to do central air systems to have AC, in fact I think the consensus is they're less efficient for individual homes and flats.

In Germany, and I assume everywhere else, the largest issue is landlords have no incentive to take care of their properties past maintaining it from a sellable perspective. Never in my lifetime has one of my landlords voluntarily made the living conditions of their property better. The moment it goes into law that homes have to be cool able down to 24° (like we mandate for heating in the winter) or landlords are outlawed (thus allowing everyday citizens to improve their homes) AC becomes a very solvable issue. I'd have installed AC in my last two flats if my landlord had let me even just take it out of rent. But no, absolutely no reason to reinvest in a home you don't live in and plan to use to supplement your income.