Whether or not you translate it depends on how central it is to the document in question (which you haven't told us anything about!)

If it is merely incidental (e.g. appears on an official letterhead), then I would say no, don't translate — in the same way that 'Dieu est mon droit' or 'Honi soit qui mal y pense' are not usually translated on UK documents.

HOWEVER, if for some reason this motto is more central to your document, then you would certainly do well to consider translating it, or at least adding your translation in brackets after it — e.g. if someone were quoting or referring to the meaning of the motto.