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[Spelos]

You are closing your <LANGUAGE> tag before you close the last <CATEGORY>

Spoiler below!

<LANGUAGE>
<CATEGORY Name="CustomStoryMain">
<Entry Name="Description">A rumbling noise has woken you up. You spring from bed and get dressed. Now it's off to the main office.</Entry>
</CATEGORY>

<CATEGORY Name="Messages">
<Entry Name="Chapter1Name">Chapter one: Down below</Entry>
</CATEGORY>

<CATEGORY Name="Inventory">
<Entry Name="ItemName_Labkey">Laboratory Key</Entry>
<Entry Name="ItemDesc_Labkey">Key to the laboratory</Entry>
</CATEGORY>
</LANGUAGE>

Having children nodes tabbed to the right a little helps you notice these problems early.