Solution:
Open /HOMEFOLDER/vendor/prestashop/translationtools-bundle/Translation/Extractor/Util/Flattenizer.php
replace this (line ~94)
public static function flattenFiles($files, $outputPath, $locale, $filesystem, $addLocale = true)
{
foreach ($files as $file) {
$flatName = preg_replace('#[\/\\\]#', '', $file->getRelativePath()).$file->getFilename();
if ($addLocale) {
$flatName = preg_replace('#\.xlf#', '.'.$locale.'.xlf', $flatName);
}
$filesystem->copy($file->getRealpath(), $outputPath.'/'.$flatName);
}
return true;
}
to
public static function flattenFiles($files, $outputPath, $locale, $filesystem, $addLocale = true)
{
foreach ($files as $file) {
$flatName = preg_replace('#[\/\\\]#', '', $file->getRelativePath()).$file->getFilename();
if ($addLocale and strpos($flatName,$locale.'.xlf') == false) {
$flatName = preg_replace('#\.xlf#', '.'.$locale.'.xlf', $flatName);
}
$filesystem->copy($file->getRealpath(), $outputPath.'/'.$flatName);
}
return true;
}