Lokalise supports flat JSON files. Here is a flat JSON file example:
{"app.name":"Lokalise","app.description":"Nice service"}
You might also be interested in checking out our JSON localization tutorial (aimed towards web devs).
If you are using TypeScript and want to add autocompletion feature for your key names, you can add the following line of code to your app:
keyof typeof import ('./path/to/some/json/file.json')