This is the default German keyboard layout: the keys/key combinations you mention are set as "dead keys" ("Akzenttasten"), so you need to enter a space after them to have them displayed (which would also enable you to easily enter e.g. â or ẽ if you enter the character you want to "decorate" instead of space).
If you don't want those keys to be "dead", you need to select a keyboard layout with no (or fewer) dead keys as follows:
- Open the control center ("Systemeinstellungen")
- Choose keyboard settings ("Tastaturbelegung") (in the first row)
- Switch to the layouts ("Belegungen") tab
- Click on the little plus sign in the lower left to add a new layout. You might want to select "Deutsch (ohne Akzenttasten)" or something like "Deutsch (nur Grave-(`) und Acute-(')-Akzentzeichen)". Once you've decided for a layout, click on "Add" ("Hinzufügen").
- Move that layout to the top of the list using the button with the up arrow (^).
As soon as you have installed more than one keyboard layout, a little keyboard icon will appear in the indicator area which will allow you to switch keyboard layouts quickly. If you're happy with only one layout and want to get rid of it, use the keyboard settings control panel just remove all the other layouts except for the one you want to keep.