how to use a default MO file

Be careful: not less than is not the identfier, but indeed the real English string that should be used by default. In WordPress, you’d use it like this:

__( 'Hello World', 'mytextdomain' );

If someones translates it into de_DE, they would translate Hello World to Hallo Welt.

To answer your question: the default locale in WordPress is en_US. So you could theoretically add a en_US.mo file that “translates” your default text. However, since en_US is the default, it’s not needed nor recommended.

Let me know if you need more help with i18n inside WordPress and I’ll happily extend my answer.