This situation is "fixed" in Alpha 3 by attempting to convert activity stream contents back to the character set of the currently-viewed language. When a character does not exist, it is now correctly replaced with a "?" rather than garbage characters.
Alpha 3 also makes available a curly template function which can be used in any other locations that are discovered over time, or in future locations as needed.
For your case, the title and link to the Japanese wiki page will still be broken in the activity stream even if you eventually upgrade to Alpha 3. If your forum uses Japanese characters, you should change your ISO-8859-1 in your language settings to an ISO-8859 compatible Japanese character set like 2022-JP, which should correct the activity stream issue (starting in Alpha 3) without also breaking content your users have already posted.
If you use other characters too (such as Russian characters), then your only option would be to convert your forum to UTF-8. That process is not as simple as changing the forum language setting, as you actually have to convert every value already saved to your database to UTF-8 (not the same as changing the database column's collation to UTF-8, or the database character set to UTF-8). My understanding is that you are currently storing ISO-8859-1 encoded data in UTF-8 fields; you would want to store UTF-8 encoded data in UTF-8 fields. If you go that route, do NOT include VaultWiki data in the conversion.