Messages in multiplayer are translated by host before they are sent to client

this is known.

One way to fix this, would be to put the messages in numbered arrays and than only send the numbers -> read + translate the strings from the arrays.
however it's a bit more complex, as some strings are more complex, e.g. a string in a string, so we would need a bunch of new network commands.

This was recently rereported. A short summary of duplicate bug 807907:

Currently messages by the host are translated on host side using the hosts localisation.
First i thought we should create a table with id's an messages so the host would only send the id and the clients would read the matching message from the list and translate it. Anyway this has two disadvantages:
1. The code gets less readable and you would have to search through the list every time you want to check what kind of message the host sends at a specific place. Using enumeration might help here. You would still have a table of messages, but the source code would look something like send_message(player_joined) instead of send_message(3).
2. If host and client use a different version (might be even if only slightly), the list may be incomplete and therefor some messages would not be translateable - e.g. if a new "sorry you use a different version, you can not play on this server" message would be added ... ;)

Therefor it's perhaps a good idea to save the localisation of all clients on host side, translate the messages (if possible) on host side and send the translated messages to the clients. if the clients use different localisation and a message is system wide, the host should keep the message in english.

Btw, comercial games have the same problem. The blizzard games for example send localised strings over the network; so if I play with my polish buddy and someone drops out of the game, I get a polish message from him (when he hosts the game).

Translating on the server might not be possible as some people will only have localized messages for their own language. I agree that there could be problems with a lookup table, but one could also do something like that:

1) host sends id of msg to display (including english text, then points 4 is not needed)
2) if id is known, client will localize it and display it
3) if id is not known, client ask host for english text (or simply displays it, if it was send in the original msg)
4) host sends english string and client displays it instead (maybe also showing a warning that the versions are not aligned).

concerning #3: this seems like a good idea, however what about messages with more parts? like:
format("Player %s got disconnected because of %s!" % PlayerName % Reason()).str(); ???

I would strongly suggest to limit those cases. For example, the only "argument" I would allow for strings is the player name. That means that all possible 'reasons' for disconnecting should get their own message and obviously a "reason not full clear" message. The communication protocol could then make a distinction between messages with argument and without. This keeps things a little less convoluted.

Fixed in rev. 6239 of lp:~widelands-dev/widelands/ggz-replacement .
Marked this report as Fix committed, as that branch will soon be merged to trunk.

The reason why I pushed that to that branch instead to trunk simply is, because a NETWORK_PROTOCOL_VERSION increase was needed, as well as by the other changes in that branch - therefore we just need one increase...

Released in build17-rc1.

This has regressed.

I can't reproduce the problem any more, so setting this back to Fix Released.