Language of the Cited Material
There is no place in which to report that the cited article is written in a particular language. A citation (<element-citation> or <mixed-citation>) may take an @xml:lang attribute, but this names the language of the citation, not that of the cited work. Parts of a citation (such as the <article-title> and <source>) may be marked with an @xml:lang attribute, but this is no guarantee that the language of the original text has been named. Some publishers insert a phrase in the text to indicate the language of the cited material (for example, “In Japanese”). Such a phrase can be captured as a <comment> (<comment>In Japanese<comment>) with a @content-type attribute indicating that this is language information (“original language” or similar).