It`s quite simple, I actually did this in Bubble back in December 2015 but now there are offline.js plugins that you can use, just search the plugin directory.
If you require another language you just have to download and customize the JS file and let it load with your site.
Don`t get confused, offline.js doesnt make your whole app an offline one. It just stores requests and warns the user if he is Offline and tries filling out forms or data. Bubble does not warn of this today, and you risk for example a user adding 1 new customer to the CRM, not realizing the internet was down, the bubble app looks like it stored it fine. She closes her computer, and the customer was never stored.
Here is my original thread on this issue: REQ: Frontend performance optimization