Excuse me, I can't speak English. This is the letter which my son wrote to you, Thank you!

不好意思，海关人员，我不会说英语。这是我儿子写给您的信，请过目。（过海关用的）

youdao

I can't speak English fluently as I am out of practice.

我不能流利地说英语，因为我已荒疏了。

《新英汉大辞典》

I can't speak English very well.

我的英语无法说得很好。

youdao

And Mr Buckles admitted: "I can't say categorically that all the guards recruited by G4S for the Olympics are able to speak fluent English".

BBC: G4S 'regret' over Olympic staffing