Prior to becoming the first (and so far only) military targets of atomic bombs, how well-known were Hiroshima and Nagasaki outside of Japan, and (if at all) for what?
Good question. I decided to look up the cities in an encyclopedia from the late 1920s to early 1930s. (You would buy the books, a total of 23, over a longer period of time, which is why the year differs between the different books.) The encyclopedia in question is a Swedish one, Nordisk Familjebok, Nordic Family Book. Here's a rouch translation of the articles. For consistency, I've written them in present tense.
Hiroshima has a population of 195,731 in 1925 and is located on the south west coast of the Hondo island. (Modern name: Honshu.) Hiroshima is an important military city, which has a military academy and headquarters. Its port district is called Ujina. Hiroshima is known for its production of cotton and silk, and trade of indigo dye, oysters etc. It's also mentioned that the holy island Itsukushima is located in Hiroshima Bay.
Area: 4,116 km². Population: 1,163,945 in 1925. Located on the west coast of Kyushu, at a gorgeous bay which forms a natural port. One of Japan's most important ports, which opened for international trade in 1854. Great shipyards, Mitsubishi Co. Export of coal, (mainly from Takashima island,) iron, steel and cotton products. Nagasaki has a medical academy established in 1920. It is also mentioned that the encyclopedia has an article about the Dutch trading post Deshima.