There are two ways to do this:
If the language you want isn't included in the drop-down list, look up its 3 letter code in the language list linked below. The language code is usually the first 3 letters of the language, eg vie for Vietnamese, ara for Arabic.
In either keyword or advanced search, enter your search words then add the following: and lang:langxyz, where xyz stands for the 3 letter language code. eg, a search for short stories and lang:langvie finds short stories in Vietnamese