I didn't get enough sleep last night, so I might say something off track if I try to speak English. Maybe we should speak Japanese today. Is that okay?
この場合、次のような言い方ができますよ。
ーI didn't get enough sleep last night, so I might say something off track if I try to speak English. Maybe we should speak Japanese today. Is that okay?
「今日は寝不足だから英語を話したらとんちんかんなことを言うかもしれない。今日は日本語で話すのでも良い?」
to say something off track で「何かトンチンカンなことを言う」
ご参考まで!