So if I get it right, "Other time:" is the expected outcome here, but the rest should not be in zh_TW.

./includes/specials/SpecialBlock.php:

'other' => $this->msg( 'ipbother' )->text(),

ALL TWO drop down menus are affected!

The reason list is in content language, because the resulting log entry should be readable on wiki.

The entries from the expiry list also in content language, because there are read from message Ipboptions and the default from ipb-default-expiry is also read as content language to reflect the wiki default.

Not a good idea to read in user language here.

OK then either show it in his favorite language and add a warning that he should leave a comment in the site language... indeed that warning would be needed on every comment box or even edit box on the whole wiki... unless we assume the user has common sense...

Or show it in the site language but add a note visible to the user saying what is going on ...

Or also have the fixed items he clicks be converted to the local language...

In any case there should be a message there saying why this is done on purpose and 'calm down, MW is not broken."