How can I customize the language used in custom installer pages?

You can customize the language used in custom installer page messages using the <customLanguageList> tag, as in the following example:

<customLanguageFileList>
     <language>
         <code>en</code>
         <encoding>iso8859-1</encoding>
         <file>pathToLanguageFiles/myapp_en.lng</file>
     </language>
     <language>
         <code>de</code>
         <encoding>iso8859-1</encoding>
         <file>pathToLanguageFiles/myapp_de.lng</file>
     </language>
    <language>
         <code>fr</code>
         <encoding>iso8859-1</encoding>
         <file>pathToLanguageFiles/myapp_fr.lng</file>
     </language>
</customLanguageFileList>

myapp_de.lng should be a text file with a format similar to this:

database.title=Datenbank auswalhen
database.explanation=Standard Datenbanken
datatabase.description=Welche Databank?
.....

and myapp_en.lng:

database.title=Database Selection
database.explanation=Default database
datatabase.description=Which database?
.....

and so on with the rest of custom language files. Then, you can for example create a <choiceParameter> page and use:

<choiceParameter>
   <ask>1</ask>
   <default>oracle</default>
   <description>${msg(database.description)}</description>
   <explanation>${msg(database.explanation)}</explanation>
   <title>${msg(database.title)}</title>
   <name>database</name>
   <optionList>
     <option>
       <value>oracle</value>
       <text>Oracle</text>
     </option>
     <option>
       <value>mysql</value>
       <text>Mysql</text>
     </option>
   </optionList>
</choiceParameter>