Fix localization for number of seconds in spinbox

Summary:
BUG: 399764
FIXED-IN: 18.12.0

Test Plan:
Works for English::
```
0.5 seconds
1 second
1.5 seconds
2 seconds
2.5 seconds
[etc]
```

Reviewers: #localization, #spectacle, aspotashev, aacid

Reviewed By: #localization, aacid

Subscribers: stikonas, sdepiets, kde-i18n-doc, aacid

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D16186