I've done FE 6 in Japanese twice, so I know I could play any of the GBA games in a foreign language. I do know a very limited amount of Japanese though, so I can pick up when they are taking about characters and places if I care to pay attention and take a few extra minutes to look at things. Though I prefer to simply read a translated version of the script. (Thanks to the wonderful souls who do know Japanese and choose to spend time translating for other unfortunate souls) I can even pick up/recognize what the different commands look like after some time.
In fact, I think I know the general control scheme of most FE games well enough that I could at least play them in Japanese. Sure I'd make a few mistakes, but I accidentally click things in English too. Awakening might be the most difficult because of all the new commands from the pair up system, but I bet if I tried the muscle memory would take over. I am pretty confident I could play FE 6-14 in Japanese if I wanted too--definitely not perfectly, but I could play them fairly normally (whilst understanding very little).
Probably couldn't do FE 1-5 as I am completely unfamiliar with them.