All three label trig buttons in lower case. It seems evident KCalc should, too.
I would prefer showing the base instead of lg, but don't know if it would work with all font sizes. In the statistic and numeral system modes, the buttons all make sure the text has a maximum of 3 characters (e.g. Mea instead of Mean).
lg is, in fact, the ISO standard. However, log_10 is the least ambigious. The length looks fine. We have a x*10^y after all.