Renaming special property
Well it will definitively cause problems for the wikis which already use the special property. It's there for over a year now so chances are high. Additionally special properties, datatypes and namespaces are currently not localizable via translatewiki.net, but this is another issue.
I'd like to point out that this is not only problem of one localisation or one property. This (and potentially some other) name for the special property is not specific to wiki or SMW in any localisation and therefore is bound to cause conflicts. There are some production SMW sites for quite a long time already.
In my humble opinion, the best solution would be to work out a policy for naming special properties that would prevent conflicts--like, adding the word page's or file's, or even prepending a double underscore to all names, while giving to those already quering special properties the opportunity to rename them in LocalSettings.php.
I think a general recommendation on how to name properties will be of great help here. Property naming tends to be a total mess. From experience I would probably never create a custom property just named "Creation date" as you yourself would not in the meantime since it is destined to conflict not only with special properties but other possible custom use cases on a wiki. I myself did occasionally fall flat on my face, too. However starters do not have this experience an you cannot blame them at all, so that's the issue I believe.
I have to disagree. In my opinion, Creation date is a perfectly legitimate name for a user-defined property and therefore not so for a special one. There was nothing in that name that suggested it would one day be occupied by a newly created special property. An SMW user can follow any agreement on naming properties or follow none; this is his business. As long as he does not invade the area reserved for special properties (and such area has never been defined), he is right. That's why it would be a good job to clearly separate names of special properties from user-defined ones. An underscore or two as I suggested above would do.