Retro compatibility issue with XWikiPreference based translations and fallback

There is a behavior difference between old and new system for which I was not really expecting to get real reports about.

To summarize (for example with "fr_FR" locale):

in old system it was doing:

• look at document content associated with "fr_FR"
• only if there is no "fr_FR" version of the document look at default document version
• look at document content for the configured default language
• look at resources for "fr_FR"
• look at resources for "fr"
• look at resources for ""

and in the new one:

• look at all bundles ordered by priority for explicit "fr_FR" translation
• look at all bundles ordered by priority for explicit "fr" translation
• look at all bundles ordered by priority for explicit "" translation

The broken use case that leaded to create this jira issue is:

• default version of the document contains French override of some standard key
• a translation of the document is "en" and empty (old hack to force it to directly goes to resources when current language is "en" as explained above)

When current locale is "en":

• before 4.3 you get default translation from the ApplicationResources.properties
• after 4.3 you get the French override from the document because this is the default translation and documents have priority over resources.

The way to do that in 4.3 basically is the opposite:

• empty default translation so that it ends up in ApplicationResources.properties
• override in the "fr" translation

Getting back the old behavior may be quite hacky. Overriding standard translation with a totally inconsistent document translation was not very clean in the first place IMO and I'm not sure it really worth it.