Is it bad to sleep talk in a different language when you only know English?
If it's not hurting you or anyone else, and it's not against the law, then you just carry on sleep talking in another language.
Spelling in American English is a little different from spelling in British English; both are English. In addition, many people who can speak a language fluently do not necessarily know how to spell words in that language, yet they still "use" that language. For example, a little child "uses" English but probably cannot spell the words that he or she "uses."