The default is that you need a complete list of language defines in the override file, as when there's an override, only that will be loaded.
There is an exception for the primary language file, e.g. english.php. This wasn't always there, but was quietly introduced in version 1.3.2 or thereabouts. For this file only the override file would be loaded, followed by the main file. The rationale behind this was that the primary language files were so large that duplicating them seemed excessive and often caused problems in the next upgrade, as new definitions added to the default file tended to get missed from the override file during upgrades.
However, there is drawback to this approach. First the overrides are loaded and then the default values, which fills in the gaps. However, it also causes PHP to issue warning notices for those definitions that appear in both files and nowadays these are treated with much more respect, so I doubt you'll see this approach extended further.


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