For a long time we have an open issue because LanguageTool doesn’t really support multiple languages in one document, not even single words. So if you write a German text but use the occasional English word (technical terms, names, …), that word will be considered an error.
I have now described a possible solution and its implications here: Bilingual support · Issue #766 · languagetool-org/languagetool · GitHub. If you have any opinion on this, please comment here or on the issue.
LanguageTool already supports bilingual text translations, so the description could cause some confusion here:
http://wiki.languagetool.org/checking-translations-bilingual-texts
Are you trying to provide support for Code-Switching? One of the problems here is dealing with common mistakes, where the word might occur in the second language, although should be spelt differently. Maybe the solution to this is to incorporate a translation dictionary, which could be used for suggestions or to suppress the rule. There are several of Code-Switching Corpus, including ones derived from Tweets than can be used for testing this.