Is there a way to fine tune locale settings? I have had this question for quite a long time and I hope that it can actually be done.

I live in Greece, but I mainly use my Android (Lollipop 5.0.2) mobile phone in English (US). The reason is that this is my personal preference. I feel a lot more comfortable having a UI in English.

The problem I have is that when using applications that operate based on locale settings (distance, decimal points, etc) everything is according to the US locale and that's normal.

But I live in Greece and would like, for example, Foursquare to display km instead of miles by default. Even though that's something you can change in settings, the decimal point in the Calculator app cannot be changed (US locale uses . for the decimal point and , for the thousands, while in Greek locale it is the other way around).

So, is there a way (not necessarily a user-friendly one) to fine tune these settings to match the Greek locale, while still using EN_US as the display language?

This does not answer my question. I would also be interested in manually editing the US locale, if necessary and possible.
In principle, no need to download any extra application to your cell phone to solve this problem. Try going to the region or language configuration and change from English from the United States to English from Europe, at least it worked for me like this.