I think ideally, the buttons should always display on one line, and the modal should expand horizontally to accommodate that.
However, currently everything is fixed width and height, except one button that is min-width. This causes it to render worse if the text is long. Nothing is cut off, but you have to scroll within the modal, which is confusing.
This mainly affects other languages that have more text than the English version.
This is related to bug 55487, since some of the centering techniques require explicit sizes (but we may have to avoid those techniques for this reason).