What Piwik version are you using?

I followed your steps (and variations like add with no existing goals, add with existing goals, and edit existing goal, as well as leaving different required fields blank) and can't reproduce the problem. The error message displayed in whatever language I had previously selected.

The language preference is stored in a cookie, so Piwik would have defaulted to the browser language if the cookie wasn't in the http header for the request.

I have used a fresh install of 1.2 and right now I had updated it via auto update to 1.2.1, i still see the error message in the wrong language, but only in my firefox, in a fresh install of chromium the error messages are in right language.

I will investigate further what firefox addon is the trigger for this error.

I disabled all addons via "firefox -save-mode", and then enabled one after another, every time I restarted firefox and repeated the steps I told in the ticket. But now i can't reproduce this misbehavior anymore.

My user agent string:

Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.15) Gecko/20110303 Ubuntu/10.10 (maverick) Firefox/3.6.15

So, i guess this ticket can be closed as invalid.

Ok, but thanks for reporting. Sometimes we revisit tickets when similar reports come in that collectively help to pinpoint the cause.