More detailed analysis:

$1 can be a single user or a list of users.

• Single case: Why supply $6, when we have $1 already and they are equal?
• List case: GENDER needs to be applied to each list element. For example, if we had three users in the list:
  • Petra (feminine)
  • Pitter (masculine)
  • P.T. (gender unknown) then $1, translated, had to be: Et Petra, dä Pitter, un dä Metmaacher P.T. (Note the initial capital!)

How to distinguish between single user and list of users?
It's generally impossible.

• For the languages having a PLURAL branch of "n==1", you can (mis)use PLURAL for the purpose.
• For the rest of the languages, you are at a loss.

In wikis having the parser functions extension installed, you could make local versions of the messages using parser functions, but that is not a general solution.

$3 looks like another user name or list. GENDER needs to be used on $3 or each user in $3 as well, in the same way as above.

All in all, this seems not really to work.

See also bug 25337
Maybe it can be treated in one go with this one.

Te message MediaWiki:useractivity-user_message also needs GENDER support on users or lists $1 and $3

Adding many blockers of bug 38638 to the list of "easy" bugs, to mark them as candidates for [[mw:Google Code-in]] tasks (gci2013). If you think this bug is not suitable, remove the keyword.

As all 4 messages no longer translatable, decline?

In T30617#3242846, @Liuxinyu970226 wrote:

As all 4 messages no longer translatable, decline?

Mark invalid if they were removed from the repository, otherwise leave open.