You only are obliged to vote IF you register and are listed on the polls.
EU expats must register to vote for EU elections
If you qualify (See conditions on that page) then you can register, and if you register, then you're compelled to vote.
If that's the case, then you will be required to vote in all of Belgium’s local elections too.
Compulsory voting in Belgian elections
The legal proceedings and the penalties for neglecting the compulsory
vote are regulated in the articles 209 and 210 of the Belgian
A first illegitimate abstinence is punished with a reprimand or a fine
of 5 to 10 Belgian franks (to be increased by 1990 additional tax,
that is to be multiplied by 200), meaning 1000 to 2000 Belgian franks.
Repetition of this offence will lead to a fine of 10 to 25 Belgian
franks (= 2000 to 5000 Belgian franks).
A replacement imprisonment is not pronounced.
If the illegitimate abstinence occurs at least four times in 15 year,
the elector is dropped from the list of voters for 10 years, and
during that period, he cannot get an appointment, a promotion or a
decoration of a public authority.
However, neglecting the compulsory vote is only punishable if it is
done illegitimately. The justice of peace can freely judge whether the
neglect is illegitimate or not.
Within eight days after the announcement of the elected persons, the
public prosecutor draws up the list of the voters who have not
participated in the vote, and whose excuse is not accepted. Finally,
the Prosecution Counsel decides which offences will be prosecuted.
The voters who have not performed their duty to vote, appear, served
with a summons, before the magistrates’ court, that judges without a
possibility of further appeal.
Belgian voters who repeatedly fail to vote in elections may be subject to disenfranchisement (ie not allowed to vote).