For the user language, you can specify any language you have set up on the user table on the system database. The language is used to select a set of maps, also the words used for 'Yes' and 'No', the decimal point character and the date format.
When the user logs on, the user interface manager selects the set of maps for the user's language. In a multi-user system, you might have a number of users logged on, each using your application in a different language. Read the section on National Language Support for more information on how DP4 handles languages.