Start by creating a structure with two fields. One (A) is localizable, the other one (B) is not. Create a simple velocity template showing both fields with $A.getData() or the freemarker equivalent.
Then create a web content article with that structure and insert a second translation besides the default one. You will be filling up just the value for field A, since B is not localizable.
Put a WCD on an empty page showing the article with that template. It will work in any of the languages.
If you insert another WCD in the page with a dummy text, the original template will stop showing the non-localizable field when showing a different language from default. When showing the default language it will still work.