{{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}} prints the hyphenation points given the current language patterns.
Hyphenation works in each language because it has defined the minimum number of letters to apply a hyphenation pattern. It applies at the word start ({{code|lefthyphemin}}) or at the word end ({{code|righthyphemin}}).
There is a third option to set the minimal number of letters the word needs to be hyphenated ({{code|hyphemin}}).
You can set that with {{cmd|setuplanguage}}, such as in {{cmd|setuplanguage|2=[es][hyphenmin=5]}}. This would enable some hyphenations and prevent others.