There is no standard. Obviously, the reserved words of a programming language or names in libraries are fixed, but there is nothing to stop you from using any language to express variable names or comments. It would just be advisable to have an internal standard if there is a team of people working on the same codebase.
Imagine you name a variable
colour; your American co-worker might search for places where colour is required, but they will search for
color, and not find the instances where you wrote the code.
In a German software company,
farbe might be used; but as long as everyone knows to use German words as identifiers, this will not be an issue.
It's all a matter of consistency and expectations. Company coding standards are there for a reason, as they make collaboration more seamless.
In case you did not talk about coding aspects, but instead the language used in the user interface: that should of course be properly localised, and thus be consistent across the application, preferably in the language of the user.