TYPO3 Core

According to the documentation, ll-XML supports externally-referenced locallang files:

<?xml version="1.0" encoding="utf-8" standalone="yes" ?>
<T3locallang>
    <meta type="array">
        <type>database</type>
        <description>Language labels for database tables/fields belonging to extension 'yourext'</description>
    </meta>
    <data type="array">
        <languageKey index="default" type="array">
            <label index="some-key">Some Text</label>
            ...
        </languageKey>

        <languageKey index="fr">EXT:yourext/path/to/fr.locallang_db.xml</languageKey>
    </data>
</T3locallang>

However this kind of reference is not supported anymore.