<foreign> (foreign) identifies a word or phrase as belonging to some language other than that of the surrounding text. [3.3.2.1 Foreign Words or Expressions]
The global xml:lang attribute should be supplied for this element to identify the language of the word or phrase marked. As elsewhere, its value should be a language tag as defined in vi.1. Language Identification.
This element is intended for use only where no other element is available to mark the phrase or words concerned. The global xml:lang attribute should be used in preference to this element where it is intended to mark the language of the whole of some text element.
The distinct element may be used to identify phrases belonging to sublanguages or registers not generally regarded as true languages.
Example
This is heathen Greek to you still? Your <foreign xml:lang="la">lapis philosophicus</foreign>?