{{cmd|mainlanguage|[nl]}} would be the way to set the language for the whole document to Dutch.
The default language is US English.
=== Change the hyphenation language ===
Another way of marking a passage would be {{cmd|startlanguage|[es]}} and {{cmd|stoplanguage}}.
=== Display languages and hyphenation points ===
{{cmd|currentmainlanguage}} prints the main language and {{cmd|currentlanguage}} displays the language in use (which may not be the same as the main language).
{{cmd|hyphenatedword|{language}}} prints how the word may be hyphenated given the current language patterns.