If you go to
System Preference, move under
International, and then click on the
Language, you will see a whole list of languages.
One of these languages can be set to become your Mac main language by simply drag and drop it to the first place. If it happens that you cannot find your main language on the list, you can look for more complete list inside the pane that is activated by clicking
Edit List... button.
After you’ve set your language, restart your Application opened currently, such as your web browser or maybe your Finder, and the change will be applied.
For example, I change to Japanese as my Mac main language (initially English). You can see the comparison from image above.