Simple Planner is written in Yii a PHP Framework.
In 2015 (just prior to my brain surgery), I wrote how to use Google Translator to automate translation of your Yii App: https://code.tutsplus.com/tutorials/localize-your-web-applic...
I had forgotten about writing that. But, I remembered, and almost instantly, most of my app was fully translated in 10 languages.