Universalist

Universalist lets you pair any primary language with any secondary language and write your page titles, headings, text, and lists in both. It automatically detects a visitor’s browser language, but you can also force the switch to the secondary language whenever needed. Once your two languages are selected, you can set dedicated content for each—giving every visitor a seamless, localized experience.

Source Code

You can find the full source on GitHub:
https://github.com/distantjet/universalist