Is it permissible to swear an oath "as is said in French"?

It is better for one who is proficient in Arabic to swear by it. However, if one swears in another language, such as French, and then breaks the oath, an expiation for the oath becomes due. The oath is valid whether sworn in French or any other language, but it is preferable for one who is proficient in Arabic to avoid swearing in other languages.