I mentioned Lt. Arthur Mears, of Britain's Indian Army, who in 1898 published a small English-Russian / Russian-English military dictionary. For the curious, here is his biography, in The Cyclopedia of India (1908). By that time he was already a major - and he certainly had had an interesting service record! (Born in Madras, trained at Sandhurst, sent to study Russian in Russia, doing survey work in Burma...).
A genealogy site mentions that apparently the same Arthur Mears died in 1941 in England. So he well may have served in WWI in his late 40s, and lived into the early years of WWII.