• The xml:lang attribute may be attached to any element
• Use <foreign> if nothing else is available
• Use ISO 639-2 code to identify language

Have you read 
<title xml:lang="deu">Die Dreigroschenoper</title>? 

<mentioned xml:lang="fra">
Savoir-faire </mentioned>
 is French for know-how. 

John has real <foreign xml:lang="fra">
savoir-faire </foreign>.