The flag is presented to Shōjima Masatomi (other possible readings) from his colleages at the Hailar (China) Auto Sales office, on the occasion of his joining the Japanese army. He was probably an early Japanese settler of Japan-occupied Manchuria. The other markings are names, slogans, etc...
I, too, have never seen the phrase "頑張ってね" (Ganbatte ne) on a flag like this. It does seem casual, but given the other things on the flag, that phrase alone doesn't bug me too much. Maybe written by the girl whose name is next to that phrase, so its probably gender/context appropriate.