When a user blocks another user and the last IP they used, DatabaseBlock::doAutoblock() inserts a reason localized using the autoblocker interface message:

$autoblock->setReason(
	wfMessage(
		'autoblocker',
		(string)$this->getTarget(),
		$this->getReasonComment()->text
	)->plain()
);

It doesn’t set any particular language for this message, so it will use whatever the language of the blocking user is. This may not be the same as the wiki’s content language, let alone the languages of other people affected by this IP block, which can be confusing:

It would be nice to actually store the message as the reason, so that it can be localized in the viewing user’s language when necessary; however, that would probably take a lot of effort. But at least it should be possible to make DatabaseBlock::doAutoblock() insert this message in the wiki’s content language, rather than the current user language.