Also, let oneself go. Behave without restraint, abandon one's inhibitions; also, neglect one's personal hygiene and appearance. For example, When the music began, Jean let herself go and started a wild dance, or After her husband's death she let herself go, forgetting to bathe and staying in her nightgown all day. The first sense dates from the late 1800s, the second from the early 1900s.

I'm posting this in case it helps you decide. I can't think of a good translation without a lot more context. HTH

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Note added at 3 hrs (2009-12-29 17:22:23 GMT)
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People letting themselves go in these kind of situation usually means they behaved badly; they let themselves go behaviour-wise, if you see what I mean?

Getting completely drunk then falling about, kissing married colleagues, pulling their pants down and showing bottoms to the camera - I think this is called mooning - fighting even, and other stuff in the same vein. This is the kind of behaviour that is often referred to as letting yourself go. In this case I don't think it can have anything to do with appearance. Or relaxation either -although I suppose it is relaxed behaviour in the sense that people tend to abandon all restraint :-)

I'll try to think of something and if I do I'll suggest it.
HTH