User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:30.0) Gecko/20100101 Firefox/30.0 Aurora/30.0a2 (Beta/Release) Build ID: 20140404024145 Steps to reproduce: I am often writing emails in different languages. So just start typing an email in another language than you did the time before. Actual results: The spell checker checks the language used the previous time. Expected results: Thunderbird should automatically recognize the language I am writing. This could be done by analyzing the existing text (when replying) or by just in time language recognition. For language detection Thunderbird could use hotwords – „Hello“ is more likely a English word than a German one. Or Thunderbird could do the spell checking parallel for different languages and take the one with less errors.