* Fix duplicate source string
* occured -> occurred
All resources I could find listed this as misspelled, but I wasn't too
sure…
Also, it should be checked if all changes to the .po-files are respected
in the corresponding src-files.
* Check that vouchers selected via API are for the correct event
* Choose different subject for reminder mails if auto-expiry is off
* correct english typos
As with PR #660, it should be checked whether the changes to the
.po-files are respected in the corresponding src-files.
