LanguageTool is an open source proofreading (spellchecking and grammar checking) service for many languages including English and German which can be run as a server providing an JSON API.
pyLanguagetool is an Python library and command line interface for Languagetool
pyLanguagetool can be installed from pip.
$ pip install pylanguagetool
Then pyLanguagetool can check text froum various sources and supports HTML, Markdown, reStructuredText and python notebooks:
$ echo "This is a example" | pylanguagetool $ pylanguagetool textfile.txt $ pylanguagetool < textfile.txt $ pylanguagetool -c # get text from system clipboard