Examples
In the opening <html> tag
The attribute in the opening <html> tag specifies a human language for the document overall.
Japanese for example:
<html lang="ja" xml:lang="ja">
or
<html xml:lang="ja">
English:
<html lang="en" xml:lang="en">
or
<html xml:lang="en">
American dialect of English:
<html lang="en-us" xml:lang="en-us">
or
<html xml:lang="en-us">
French:
<html lang="fr" xml:lang="fr">
or
<html xml:lang="fr">
Canadian dialect of French:
<html lang="fr-ca" xml:lang="fr-ca">
or
<html xml:lang="fr-ca">
German:
<html lang="de" xml:lang="de">
or
<html xml:lang="de">
Korean:
<html lang="ko" xml:lang="ko">
or
<html xml:lang="ko">
Etc.