Harrap's.

in France, the word is archaic; it referred to a municipal magistrate
for alderman, I've found: officier d’état civil = registrar (of births, marriages and deaths);
hope it'll help!

this roughly describes the job that an échevin does. I find "alderman" as bit out-dated as a term in UK English.

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Note added at 10 hrs (2011-11-17 01:51:56 GMT)
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Hal, Phil and writeaway raise some interesting points which I can't easily address in the 255 characters I have to reply to each of them! Basically, why did I suggest "Deputy to the Mayor" and not "Deputy Mayor". After all, there is a Deputy Mayor of London: http://en.wikipedia.org/wiki/Deputy_Mayor_of_London

Well, there are at least three reasons for my choice, which I admit is a personal one and not a matter of 'correct or incorrect'.

1. Eight years living in France has immunised me with the term 'député-maire' (transl. "Mayor and Deputy"), which is rather different to 'adjoint au maire' (transl. "Deputy to the Mayor")

2. The term "Deputy Mayor" exists in UK English, but it is not well known and so is not an 'obvious' translation

3. In UK English, there could only be one Deputy Mayor for a given place (most places don't have one at all, as far as I can gather). This is just the standard usage of Deputy XXX across all public and professional offices (in the UK: US usage is slightly different). However, there are usually several 'adjoints au maire' in France or 'échevains' in Belgium, and so the avoidance of ambiguity demands a different translation.

4. In this specific context, it is the responsability of the Mayor (in France or Belgium) to register births, marriages and deaths in his or her municipality. In most places, the Mayor delegates the routine responsibility, and the person who takes it on is then *deputising* for the mayor in that particular function.

Thanks for your comments, you've made me think, but I still stand by my suggested answer as slightly better than "Deputy Mayor" in this context. For me, "Deputy Mayor" is 'premier adjoint' or 'premier échevin', not the person who we usually get on birth or marriage certificates.