XML doesn't support arrays with numeric keys. So you need a normalizator for this (again):
````php
->beforeNormalization()
    ->ifTrue(function ($v) { return isset($v['fallback']); })
    ->then(function ($v) {
        $fallbackLocales = array();
        foreach ($v['fallback'] as $fallback) {
            $fallbackLocales[] = $fallback;
        }

        return $fallbackLocales;
    })
->end()
````