Hi,
having not much background in Alfresco, but a lot of background in search, here my suggestion:
<ul>
<li>if the user selects several categories within the same category, they should be connected with OR (e.g. user selects languages/English and languages/German => they most probably want documents taht are German or English, not documents which are English and German at the same time)
<li>if the user selects several categories contained in different categories, they should be connected with AND (e.g. user selects langages/English and region/EMEA => they most probably want documents in English language from EMEA, not all English documents from any region plus all documents from EMEA in any language)
</ul>
(assuming a 'properly built' category tree – where the subcategories are actually subcategories)