When changing a language file in acp, the wrong file is selected:

When you choose viewtopic.php in the selectbox, you press select, the default file (common.php) is selected.
If you are not sending it to the ftp, there is no problem.
BUT, when you send it to your ftp, the forum thinks you are editing common.php, instead of viewtopic.php, and save it as common.php, so your common.php is overwritten by viewtopic.php.