If your example was working correctly, it would look like this
(By the way I'm assuming that the spacing shown is just the effect of pasting it here, and that it's aligned normally when it displays in your browser.)
So it isn't "displaying both the define text and the actual text", it's just that the define label appears, rather than its value. Don't worry too much about the rather strange conditional, you're in the $topic_depth == 'articles' branch. But I still can't tell you why it's happening, since your language file looks just like mine. Ummm, and if it couldn't access that language file I'm pretty sure it would give you that annoying T VALUE error message instead.
In your article_listing.php, can you check the code that displays these lines? E.g. for TEXT_ARTICLES at about lines 35-42, it should look like this:
?>
<tr>
<td class="main"><?php echo TEXT_ARTICLES; ?></td>
</tr>
<tr>
<td><?php echo tep_draw_separator('pixel_trans.gif', '100%', '10'); ?></td>
</tr>
<?php
If you reply, it would be helpful to know what you were doing immediately before this problem started. Did you just install Article Manager? Did you make any changes to the code, if so, where? Directly in the database? Editing existing article/topic text? Inserting a new article in a previously working topic?