I think the easiest is to define the et-al term without a period in the beginning of the style <locale> <terms> <term name="et-al">et al</term> </terms> </locale>
I think it might be possible to also do using strip-periods, but I'd have to test.
thanks, @adamsmith for your comment I have tried strip-period however it doesn't work maybe I hadn't use it in the right place. However, I'll follow your suggestion to define the et-al term and I'll let you know if it works
<locale>
<terms>
<term name="et-al">et al</term>
</terms>
</locale>
I think it might be possible to also do using strip-periods, but I'd have to test.