If I send a text message from my android to an iphone user who is overseas and their phone is in airplane mode,
but wifi is enabled, will they get my text?
They won't since SMS uses the cell signal to receive messages. You would have to be using a 3rd party app (like WhatsApp) or something. iPhones can use their iMessage feature but this is an "Apple only" thing.
I think not unless you use one of the Internet texting apps. For instance in US if I turn off data I can still send and receive texts. It I go airplane mode then enable wifi texts don't work. If I use an internet based texting app like TextNow the exact opposite is true.