Is there a short or casual way to say "My Japanese isn't good"? At the moment I say something like "watashi no nihongo wa ii jya nai" but I know that isn't good Japanese. Gomen ne ^_^'
How to explain poor Japanese?

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Boku wa Nihongo ga heta desu. I am bad at Japanese.
Nihongo ga zen zen dekinai. (My) Japanese is not good at all.

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if you want to be super polite, you can say
Nihongo ga sukoshi wakarimasu, demo, jozu ja arimasen.
:)

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"Boku wa nihongo ga jouzu ja nai" 
"Boku no nihongo wa mada umaku nai." 
You can change "umaku nai"&"jouzu ja nai"

"Boku wa nihongo ga heta desu." = "My Japanese is bad"

formal＜ー　watashi - boku - ore　ー＞to be close

from poor Japanese man:P