Reported for: WooCommerce Multilingual 3.9
Resolved in: WooCommerce Multilingual 3.9.1
When using WooCommerce Multilingual 3.9, the stock quantity is not updated across all translations of a product when an order is placed.
The only workaround is to set the manually set the stock based on existing orders or to revert to the previous version: 3.8.6
Note: In the backend, saving the product in the original language, does sync the stock with existing translations.