LanguageTool has always supported several languages, but it doesn’t have real support for mixing languages in a document. As a first step to improve that, I have now activated the “alternative languages” parameter for LT’s API. It is implemented on languagetool.org like this: if you check a non-English text, but your browser accepts English (see below), English words in your text will not directly be considered an error, but they will be marked with blue:
This is only a first step, although I cannot make any promises about the next steps towards supporting multi-lingual documents.
To see whether your browser accepts English, visit its configuration dialog.