lang() third parameter: bug or feature?

Hello.
If i use this once:

PHP Code:

lang('some_file.some_string', [], 'fr'); 


This will result all next:

PHP Code:

lang('some_another_file.some_another_string'); 


will show last used locale string as if we had done so:

PHP Code:

lang('some_another_file.some_another_string', [], 'fr'); 


It's ok?

At least this behaviour is not documented. I just ran into the same issue. I think that this is a bug.