Appends yaml localisation files instead of replacing when included in the vfs
Otherwise it could lead to copyright problems when developing a patch mod: The current system requires to include the original localisation file (or the end user should manually edit the localisation files, or use an external tool,... which could be a hassle).
Appending the yaml files would solve the problem.
But it should be combined with another feature in the localisation system: Using the last localisation key of the file (the current system load the first).
Not sure if this feature is desired, but it separates the GMST localisation issue in two distinct parts (and also the "un mainainted" mod problem).