How to remove a user defined language in Notepad++

In Notepad++ open the Preferences dialog box by clicking on Settings -> Preferences click on the New Document/Default Directory tab and change the Default Language to XML.

EDIT: Just saw that you wanted it to apply to .config files as well. This can be done but you'll need to do it for every extension you want to override. Click on Settings -> Style Configurator scroll down to XML in the language section and add config to the User ext box. Save & Close and any new .config file opened will show up as XML. In fact, I've already done that for my installation as seen below:

You can define your language as you wish (and make a new programming language!), but the most convenient way is to import an XML document that defines the whole configuration. An XML file for this purpose can be downloaded from here: http://www.mediafire.com/download/qv84x0wjwn816d6/userDefineLang.xml

Then you have to define this file using your own syntax as follows:

<?xml version="1.0" encoding="Windows-1252" ?>
<NotepadPlus>
    <UserLang name="(name your language here e.g.P++) " ext="java">
        <Settings>
          <Global caseIgnored="no" />
          <TreatAsSymbol comment="no" commentLine="no"/>
          <Prefix words1="no" words2="no" words3="yes" words4="yes"/>
       </Settings>
        <KeywordLists>
            <Keywords name="Folder+">{</Keywords>
            <Keywords name="Folder-">}</Keywords>
            <Keywords name="Operators">- ( ) * , . / : ? @ [ ] + =</Keywords>
            <Keywords name="Comment">//</Keywords>
            <Keywords name="Words1"></Keywords>
            <Keywords name="Words2"></Keywords>
            <Keywords name="Words3">instanceof assert if else switch case default break goto return for while do continue new throw throws try catch finally this super extends implements import true false null</Keywords>
            <Keywords name="Words4">capsule design package transient strictfp void char short int long double float const static volatile byte boolean class interface native private protected public final abstract synchronized enum</Keywords>
        </KeywordLists>
        <Styles>
            <WordsStyle name="DEFAULT" styleID="11" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" fontSize="9" />
            <WordsStyle name="FOLDEROPEN" styleID="12" fgColor="808040" bgColor="FFFFFF" fontName="" fontStyle="0" fontSize="14" keywordClass="Folder+"/>
            <WordsStyle name="FOLDERCLOSE" styleID="13" fgColor="808040" bgColor="FFFFFF" fontName="" fontStyle="0" fontSize="14" keywordClass="Folder-" />            
            <WordsStyle name="KEYWORD3" styleID="7" fgColor="0000FF" bgColor="FFFFFF" fontName="" fontStyle="0" fontSize="" keywordClass="Words3"/>
            <WordsStyle name="KEYWORD4" styleID="8" fgColor="8000FF" bgColor="FFFFFF" fontName="" fontStyle="0" fontSize="" keywordClass="Words4"/>/>
            <WordsStyle name="COMMENT" styleID="1" fgColor="008000" bgColor="FFFFFF" fontName="" fontStyle="0" fontSize="" keywordClass="Comment"/>
            <WordsStyle name="COMMENT LINE" styleID="2" fgColor="008000" bgColor="FFFFFF" fontName="Comic Sans MS" fontStyle="0" fontSize="" keywordClass="Comment"/>
            <WordsStyle name="NUMBER" styleID="4" fgColor="FF0000" bgColor="FFFFFF" fontName="" fontStyle="0" fontSize="" />
            <WordsStyle name="OPERATOR" styleID="10" fgColor="804000" bgColor="FFFFFF" fontName="" fontStyle="1" fontSize="" />
        </Styles>
    </UserLang>
</NotepadPlus>

Then you would see your language in the language tab.