Depends on the requirements. I've had many internal-only applications only require one language: English. At that point you can hard code many things, which skips the loading other files, checking different languages, etc.

And what is this localization file called? We do not have a system where non dev can commit any change in our product.

We do have a constants file which has all the messages but a PM does not access to code/commit and has to go via a Dev.

It really depends on who‘s the target group. I hate localization in Developer Tools etc, because googling it is harder and a lot of times English is just easier to understand. It’s different for a shopping site.