Document cache key collision when page name matches a locale

The issue can be noticed on a clean XWiki instance by following these steps:

• enable multilingual and add French to the list of supported langauges
• create a new page and put this code:

  {{velocity}}
  $xwiki.getDocument('Sandbox.TestPage1.fr').documentReferenceWithLocale
  {{/velocity}}
  

Surprisingly, the output is: xwiki:Sandbox.TestPage1(fr). The page name "fr" is treated as a locale. The issue is in UidStringEntityReferenceSerializer which is used to compute the cache key. The problem is that the locale is appended like any other reference component.