In the "Select and Mask" dialog the default keyboard shortcut for "Decrease Brush Size" on german keyboard layouts is not working. The default key is the "Ö"-key. It gets registered as the "O"-key and therefore changes the view to "onion skin" instead of changing the brush size.
Thanks for letting us know. I can confirm the bug that you're seeing and it appears to impact all languages with diacriticals for these shortcuts. I've logged the issue to be fixed.
Sorry for the inconvenience. A work-around for you would be to set your GLOBAL Brush Size shortcuts, found under Edit > Keyboard Shortcuts and then Tools, to keys that are unique and which lack umlauts or other such marks.