calendar: dialog - internationalizing title and warning messages

[andreaschu]

The warning messages as well as the title are not internationalized yet.

• at least the warning messages depend on issue #533.

[vdick]

@andreaschu warning messages and button label are implemented (should works after next build and rollout). What about the Title? Will it remain the current one? ("Aktivität")

[andreaschu]

works!