Testcase:
Open the preferences panel. In the timer section, click the "restbreak" tab.
Now, click the "daily limit" tab.

Actual result:
The widgets for entering break time and the checkboxes to their right appear to
jump to the left, because the text "pauze tijdsduur" in the restbreak tab is
longer than the text "herstart om" in the daily limit tab.

Expected result:
The widgets should remain at the same location.

Fixed. Now, if anybody would tell me how I can fix stuff like this in
Java/Swing, I could do a good GUI at work...

Hmm...
Can't you write your own layout manager??

What I would like is to have the standard layout managers satisfy extra
constraints, like "do a normal gridbag layout, ~and~ match the width of this
widget to that width of that widget". 

If I write my own layout manager, I cannot use the standard layout managers
anymore. I am not sure if I could write my own layout manager that delegates to
existing managers while at the same time satisfying my constraints...

Btw, the time widgets still jump, though this is intentional.... if I don't have
them jump, the spacing between the label and the time entry becomes rather
large, especially in other translations where the difference in label length
varies more...

Strike that last paragraph, you convinced me...  
Now, widgets aren't jumping at all.