I understand your concerns but let me give background on the motivation:
We were hoping to target this mode more to developers and long term wikipedians. Ideally we are trying to create a sandbox where we can encourage experiments from developers to try out editing ideas in the wild and to get rapid feedback without bothering the product team.
Do translations need to be word for word translations - I was hoping there were similar ideas/idioms in other languages that could be used to encourage the right audience to the alpha? For instance in English Brandon also had the name "The proving ground" (http://en.wikipedia.org/wiki/Proving_ground). Worst case if no such phrase works, I could imagine some languages simply using alpha. It just seemed an opportunity to take to attract the right kind of people.
I feel that the length of the message is not so important - the only 2 places it shows up is in brackets in the search bar (only a temporary measure - soon it is hoped this will be replaced by some kind of branding) and the settings page...
I understand your concerns I just want to fight this corner as I want to make more people smile!
Most translators work on many strings of many projects they have no clue about. They just translate strings aiming to leave their % of completion as high as possible. Asking them to come up with alternatives to "Here be dragons" is asking a bit too much. Why not calling it "Lab" and be done with it? Most languages have an equivalent for that.
PS: I don't feel like fighting this corner. You were looking for feedback and now you got some. :)
An easy improvement is to remove the link to Wikipedia from the message and move it to the message documentation: 1) native speakers of the language in question mustn't need it or your choice of language is wrong and it doesn't have to be replaced with an equivalent in other languages (or even worse, to link English from other languages), but 2) translators need it to understand what you mean. Also add to documentation that one can replace it with whatever else they feel appropriate (the point of docs is precisely to address the "many projects they have no clue about" issue).