I am assuming that you are translating a Belgian patent. The following US patent manual provides a lot of useful information on foreign patents. Here is a section on Belgium.

3. Belgium. Belgian patents are normally granted within a month from filing, after a minimum formal inspection; in some cases the granting is delayed until six months after filing. The granting of the patent consists in the making up and signing of a "ministerial decree' or departmental order, which remains in the file; in due time the applicant (now patentee) is sent what amounts to a certified copy, with an attached copy of the specification. In the normal case the specification of the patent is kept secret for a period of three months after the patent is granted, by a specific provision in the statute, after which it becomes available to the public. Such a period of secrecy is tacked into the granting date and the date of a Belgian patent as a patent to be considered in using it as a reference under 35 XJ.S.C.102(a) and (b) is taken as the date when the specification became open to public inspection. In re Ehenstam, 1968 CD. 402; 118 U.S.P.Q.349. (A period of secrecy occuring after grant has not been taken into account in connection with 35 TJ.S.C. 102(d). There is no period of secrecy after granting in the delayed issue cases and the effective date as a patent for these cases is the actual date the patent was granted. The specifications are not issued in printed form until about two or three years after the patents are granted. The printed copies received since No. 620,001 (granted January 10, 1963, available to the public January 10, 1963, printed February 2, 1965) consist in a reproduction of the granting Order as the top page, together with a reproduction of the specification and drawing. The filing date appears in the preamble of the Order; the patentee's name and the title of the invention, and any claim for priority, appear in Article 1 ; and the granting date is at the end above the official's signature. The date on which the specification became available to the public appears in the upper right hand corner in the lower part of a rectangular frame, below the expression "Brevet mis au lecture" in the French language copies. This is the date to be used for citation purposes and as the effective date as a patent. The date on which the printed copies were issued as such does not appear on the copies, but can sometimes be ascertained from the title page of the bound volumes. The copies are received in groups of 50 already sewn together for binding, with a title page giving the date on which they were printed and published. (see page 13 of the PDF, which is unfortunately not searchable)
http://www.uspto.gov/web/offices/pac/mpep/old/E4R0_900.pdf

I think it would be safe to use "published on", "publication date", "date of publication", etc.