I discover yesterday what I think is an important issue for users living in Canada and speaking French (like me by the way).
When a Language Pack is installed on a SharePoint 2010 Farm, the browser language is detected by the SharePoint sites on which MUI is enabled. To detect the user language, SharePoint inspects the HTTP request which contains the header "Accept-Language". The value of this header can be configured in the Options of your browser. Talking about Internet Explorer you have to open "Internet Options" --> "Languages".
Thus, if you install the French Language Pack and if your browser is configured to accept Fr-Fr (French - France), your SharePoint site will be displayed in French even with a site created in English.
So what's the problem? This detection does not work with the locale "Fr-Ca" (French - Canada). It seems that SharePoint displays the correct default language only if the locale set in the browser matches exactly the locale of the SharePoint Language Pack. Of course there is only one French Language Pack and its locale is Fr-Fr.
This is very annoying for french users since the SharePoint sites will be displayed in English by default. They have to switch manually to French.
Fortunately the workaround is pretty simple : add the Fr-Fr locale in the languages of your browser. You can set it in second position after Fr-Ca. Of course you will have to deploy this setting to the users :)