Update topic_preview_acp.php

[David-NF]

I'm not sure how to translate this:

'TOPIC_PREVIEW_DRIFT'           => 'Animated drift effect (in pixels)',
'TOPIC_PREVIEW_DRIFT_EXPLAIN'   => 'Amount of vertical animation on fadeout

I tried to figure out what this function does, but I see no effect when I change the values

[iMattPro]

it's the amount of vertical movement up or down that occurs when the topic preview disappears.

[David-NF]

Interesting, I barely saw it... but now I noticed and translated the last 2 language variables