don’t, won’t, it’s, can’t, there’s, …
do not, will not, it is, cannot, there is, …
In general, contractions (such as the examples above) should not be used when writing user-interface strings, for two main reasons:
contractions are avoided in formal writing
they make English text harder to understand for non-native readers (and although KDE has great translation teams, the original English string is used if no translation exists)
For further discussion on this issue, in general, see the following blog post: Don’t Use Contractions?