I have seen soooo many translations on flags. Try looking some up on here on the threads and you might find one similar. Either way someone will most likely reply to you in a lil bit sorry I cant give you much since I know nothing about it, but just a suggestion i guess
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.
Well if he was joining in the earlier days then the occasion would be more light hearted and optimistic. I mean I don't know but I was always assuming that when these flags were written the general assumption was that the recipient very likely would never come back and it wasn't a very happy thing. But that's probably not correct.