Installer: Put the translated custom messages in a separated file.
Translation script: Use the lang short name instead of the full filename.

The filename depends only of the language short name and we will need the short name later anyway.
Translation script: Split the main script into two files.

Separate functions directly related to PO(T) files from those directly related to RC files.
Translation scripts: Add support for the installer strings.
Transifex client config: Add the new resource for the installer strings.
Fix a possible dead lock when opening a file when one is already playing.

Easily reproducible with madVR.

Ticket #3865.