I would like to say "It is written half in English and half in Japanese."
When writing this myself I came up with:
but I think that
does not sound very natural?
I think that
is fine. I think you could also say:
半分 like that is completely fine. You can use other fractions as well (but in that case, you cannot use it adverbially and have to insert the particle
は after it like
三分の二を). But if you do not particularly need to emphasize the act of writing, simply
書きました is more natural than