Hi lilmisssunshine,
My son is bilingual in German and English. So our languages do have the same letters - not quite what you are asking for but I thought I'd chime in. Like Japanese German is very easy for learning reading - once you know the letters and you get the idea of blending you can read almost anything. There are a few exceptions, specific rules but those are quickly learned.
It was my plan to teach my son to read in German before he'd learn to read in English (he knew the letter sounds in both languages very early). When he was 4 he suddenly spelled all kinds of German words to me. I was stunned. So I asked him to read and he could read. We waited a bit and then helped him learn English.
So from my limited experience I would suggest the following: Teach the easy language first. That way your child will learn the concept of blending and then can apply that to English and learn the different English phonemes.
I actually think it might be easier, btw, to have different letters! The trickiest thing with learning to read in German and English almost silmutaneously was that the exact same letter makes totally different sounds. That definitely held him back.
Good luck!
Stefanie
