[译] 他总是用英语给朋友们写信。
[误] He always writes letters to his friends with English.

[正] He always writes letters to his friends in English.
[注] 使用某种语言，应该用介词 in；使用某种工具，应该用介词 with. 如：He writes letters in English with a pen. 他用钢笔写英文信。