`Locale::getDefault()` will give you the *default* locale, which won't match the configured one... So I'd suggest:

```php
public function __construct( TranslatorInterface $translator, array $suffixes, ConfigResolverInterface $configResolver, LocaleConverterInterface $localeConverter )
{
    // ...
    $this->configResolver = $configResolver;
    $this->localeConverter = $localeConverter;
}

public function sizeFilter( $number, $precision )
{
    // ...
    $formatter = new NumberFormatter( $this->getLocale(), NumberFormatter::PATTER_DECIMAL );
}

private function getLocale()
{
    foreach ( $this->configResolver->getParameter( 'languages' ) as $locale )
    {
        $convertedLocale = $this->localeConverter->convertToPOSIX( $locale );
        if ( $convertedLocale !== null )
        {
            // Setting the converted locale to the _locale request attribute, so that it can be properly processed by parent listener.
            return $convertedLocale;
        }
    }

    return Locale::getDefault();
}
```