Alright I've modified it to what I really need:
Here's a Screenshot of the Area that is supposed to work but doesn't since the extra_english.lang file doesn't seem to work. (Note that the upper (not marked) Sign Area in the Screenshot works with the Code added to the original english.lang file of the main story).
And here's the code itself, while the key description + name and the area won't show:
<LANGUAGE>
<CATEGORY Name="Inventory">
<Entry Name="ItemName_key1">Private Study Key</Entry>
<Entry Name="ItemDesc_key1">Unlocks the door to the Private Study.</Entry>
</CATEGORY>
<CATEGORY Name="Signs">
<Entry Name="TestSign">This is for testing purpose</Entry>
</CATEGORY>
</LANGUAGE>