Source and more information: SeaMonkey Project Blog
today I switched from Build 20190329130005 to latest available Server Installation of unofficial (by wg9s) en-US SeaMonkey 2.53 (NT 6.1; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0 Build 20190122130014 (Default Classic Theme, German Language Pack active) on German WIN7 64bit. And the first thing I saw was that message again. Strange! AFAIR I only see that on my Browser Home Page, but I am not sure.
I will ask at mozilla.support.seamonkey.
I got a new German Windows-Language Pack. More information in the German SeaMonkey Blog!
I got 2 German Lightning (calendar) versions for Windows from Frank-Rainer Grahl, you can download them here:
Unfortunately I currently have no information for which SeaMonkey versions these Lightnings are suitable,I asked frg to leave a comment here.