Keyboard shortcuts are a challenge on many international keyboards, since software developers frequently choose the shortcuts based on the physical key layout of the US keyboard.
For your specific case, in theory you should be able to access the ‘Add Inside’ shortcut with Cmd-Shift-Option-9. This works for me on a different layout where ] is accessed with Option-) .
Thankfully with native Mac apps you can remap the keyboard shortcuts for menu commands. Go to System Preferences > Keyboard > Shortcuts > App Shortcuts. Press + , select ‘OmniFocus’, enter the exact menu command label, and press a combination of keys: