NLTK (Natural Language Toolkit) is a well-known platform for Python application dealing with human language data. It includes many downloadable lexical resources (named corpora). If your application requires some corpora to work, add a nltk.txt file at the root of the application containing a corpora name per line. For instance:

punkt
stopwords

The Python buildpack will automatically download them during the deployment of your application.

Note that there is a known issue on nltk which leads to an unharmful warning message when your app starts:

RuntimeWarning: 'nltk.downloader' found in sys.modules after import of package 'nltk', but prior to execution of 'nltk.downloader'; this may result in unpredictable behaviour

You can safely ignore it.