FOREX-Lowest euro zone inflation in 5 years leaves euro weak

* Dollar index set for biggest monthly gain in over a year LONDON, July 31 (Reuters) - The euro hovered near a nine-month trough against the dollar on Thursday after data showed euro zone inflation falling to its lowest since the height of the financial crisis five years ago. Consumer prices in the 18 countries sharing the single currency crept up 0.4 percent on the year in July, down from 0.5 ...