Heimdall is an open-source tool, written in Python, that searches a text for sequences written in other languages than the main one. Heimdall's roots in historical linguistics are reflected in the language models it is shipped with: its three default languages are medieval Latin, Early Modern French and Early Modern English. The tool is compatible with plain text files as well as xml-files. In the first case, it simply outputs a list of all foreign passages. In the second, it also allows you to tag the foreign passages directly in the input documents.
You can download Heimdall from GitHub (click green button and select download zip-folder).
For more information you can visit the Heimdall wiki on GitHub.