Babble is one of the best open source plugins, as same as WordPress, Babble is completely open source.
Babble is the fastest plugin in same kinds of plugins, and in the developer’s words, Babble has been optimized the code for fast operation. As a result, this plugin reduces loading time by 20% and queries by 40%. Just download Babble from the GitHub and follow the brief instructions to install it.
Babble can translate your website content into the unlimited number of languages, you need only switch to the WordPress dashboard translation interface and click any existing post, page, or category you want to translate, then you can get the translation result.
Babble can display two side-by-side WordPress visual editor palettes, one is to display the original text and the other one is to to be blank, where you can fill in your own translation.
After filling in the translation content in the panel, place a special gadget in the sidebar so your users can read your translation content. The users only need to click a button to select his language.