This will not be in the official release of Plus! because it is seen as a dialect... This has been discussed intensively before, so I'm not going to start it again, you can search for the threads if you want thought.
Some things you may want to know/do:
The current dutch translation is already in proper dutch for people from the Netherlands as well as for people from Belgium. We've put enormeous effort in it to make the translation not typical Dutch nor typical Flemish. But, if you do find some translations not correct, then please report them here, so they can be corrected.
I doubt, there will be a big differences between the current translation and a typical Flemish version, except for some typical used words in Belgium.
Be consequent in your translation. When you use "je" , don't use "u" or even "U" in another sentence for example. This also goes for window and panel titles for example.
Bare in mind that space is often limited to show the translation.
You must know Plus! very well to know what each function exactly does. Although you'll get a hint from the english translation (or the current dutch one).
Don't forget things to translate, use
Choli's Translator Application to aid you in that.
Also, read this document first:
http://files.msgplus.net/data/TranslationGuidelines.doc
But if you want to make a typical Flemish version, then do so by all means.
PS: I myself am a guy from Oost-Vlaanderen, Belgium