Well, you can check for specific characters that English lacks - Ñ and vocals with accents for Spanish; umlauts (ä, ö, ü or ß) for German, and likely most Germanic languages; and so on.

If there are English articles AND at least certain number of non-English characters, then the text is likely in two or more laguages.

A more complex option woul counting these English articles, I guess there would be a certain ratio for them in a common English text. Say 0,8 articles per sentence in average or so. If the ratio gets below certain threshold, the text likely contains other language(s), or perhaps is not a fluent natural text, but say a table or something similar.