不自信用英语可以用“lack of confidence in English”来表示。这个说法来源于对不自信的定义，即缺乏信心。例如：

I lack confidence in my English speaking skills.

我缺乏信心讲英语。

She has a lack of confidence in her ability to learn new vocabulary.

她缺乏信心学习新词汇。

He lacks confidence in his ability to communicate effectively in English.

他缺乏信心在英语中有效地交流。

总之，“lack of confidence in English”就是指对自己的英语能力缺乏信心。