Rhasspy

Rhasspy is an open source, fully offline set of voice assistant services for many human languages (see rhasspy.readthedocs.io). Rhasspy combines all needed tools for a voice assistant, from wake word to intent handling.

Advantages

It is easy to install Rhasspy with an installation script
Unlike other voice assistants like Google, Alexa, etc, Rhasspy does not share data across the internet and this is very interesting for people, who are concerned about their privacy
Once you have chosen Rhasspy as your voice assistant, you can add additional features for your needs. E.g. it is possible to train Rhasspy to use your own voice for text to speech engine

Disadvantages

Not easy to set up. To get Rhasspy running on a device like Raspberry Pi, many settings are required. In most of the cases, you can find problem solving on different websites, but they are not always updated. You need some more effort to get the correct setting

Conclusion

Rhasspy is an easy-to-install control centre for various software, which together enable an offline-capable voice assistant.