Finally I managed to update my language files, previously I updated only PO/MO in the wp-content/plugins/wpforo/wpf-languages/ folder from the plugin but this had no effect as the languages files are copied into wp-content/languages/plugins/ folder.

If I copy the updated languages files PO/MO to wp-content/languages/plugins/ folder then after a clear cache the labels are now OK!

But this is strange why there is no change done by Loco Translate plugin while it is the case for other plugins...

Definitely there is something unclear with the language configuration in your wpForo flow (xml, PO/MO, etc...)