i guess you want to say how poor your Japanese is. so in that case, well, there are many ways to say it, but for instances "watashi no nihongo ha(is pronounced as"wa") zenzen damedesu" or "watashi no nihongo ha totemo heta desu". actually your original sentence sounded a bit off. i understand why you said so, tho :) hope it helps :) good luck! :D

Japanese has many words for "very", "extremely", I put all words that come up with.... とても、とっても、すごく、すごい、非常に（ひじょうに）、かなり、きわめて、ぜんぜん(used only with negative form)、 おおいに、大変(these two used with positive form only, I think..)、 まったく、すこぶる、めちゃくちゃ...and so on..