Bug 30994: Typo: item was on loan. couldn't be returned.

Bug 30994: Typo: item was on loan. couldn't be returned.

This patch updates some language in the inventory template to make it
readable and consistent: Punctuation fixed, capitalization made more
consistent, language corrections ("check in" instead of "return").

To test you can try to apply the patch and trigger the various errors in
the inventory interface, but it's probably enough to visually confirm
the changes in the patch.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>