Preview doesn't work when translating help files because $forcelang param is filtered in help.php as PARAM_ALPHAEXT and then the '8' is removed from 'en_utf8'. Resulting $filepath is wrong:

dataroot/lang/en_utf/help/accessibility.html

> lib/moodlelib.php

>

> case PARAM_ALPHAEXT: // Remove everything not a-zA-Z/_-

> return eregi_replace('[^a-zA-Z/_-]', '', $param);