dc:Language
The basic letter codes that define which language is used in the eBook in according to http://www.openebook.org/. You can obtain list of languages at tab page "Languages support" (select menu item Options | Preferences), then mark up appropriate languages. After do this, you can select language code from attributes drop-down list but unfortunately you can not do in that style in this case.

attributes

id
Provides an element with a unique name within a single document. This is especially useful in works with extensive cross-referencing and linking, such as dictionaries, encyclopedias, or travel guides. Use of ids simplifies linking and updates.
xml:lang
xml language code.
xmlns:dc
Always equal to "http://purl.org/dc/elements/1.0/"

Sample:

<dc:Language>en</dc:Language>

See also:

<dc-metadata>