Some context to begin with: My wife and I have been married for two years now. I earn around 2500€ after taxes, and she used to earn around 1500€ after taxes. We both share one bank account, by the way.
I come from a working-class family. While I wouldn't describe us as "poor", reckless spending was frowned upon. Larger investments were discussed and alternatives sought after. Impulse purchases were limited to very small amounts (e.g. a bottle of Cola).
She also comes from a working-class family, but with far less responsible parents. Her parents divorced during her childhood. Her father, who raised her, spends money without any thought for the future or savings. This always lead to them having financial trouble at the end of each month. Her mother doesn't exhibit any care for the future either. A while ago, she took a loan for a luxury car, which she would never be able to pay off. All her money goes into that car, yet she claims that she is doing just fine.
What has happened
My wife recently decided (without asking me about my opinion), to switch from a 40h/week position to a 20h/week position, bringing her overall income down significantly.
At the same time, her spending habits have increased significantly. During may, she spent almost 1500€ on various luxury goods like perfume, shoes, clothes, etc.
When asking her about it, she responded dismissively at first, akin to "That's all in your imagination. I don't spend more than usual.". When I then showed her how much she was actually spending, she then claimed that I should "Let her have some nice things".
I'm afraid that this behavior could intensify, leading her to spend ever increasing amounts of money for unnecessary things. Furthermore, I feel a bit hurt from her assertion that I wouldn't let her have "nice things". I personally feel like spending a quarter of your monthly income on one pair of shoes leaves the realm of "nice things" and starts becoming irresponsible. It's as if I would spend 625€ on a bottle of wine.
How can I effectively communicate with her that her spending habits are unsustainable with our current income and that I'm worried it could lead to more problems down the line?