Overview of the Issue
In some cases, the emails that are sent when the status of WooCommerce orders changes, are always sent in the default site language.
This happens when using the 3.8 version of WPML and you have the Assume that all texts in PHP strings are in English setting active, on the WPMLTheme and plugin localization page.
We are currently working on fixing this.
Workaround
This issue is fixed. Updating to the latest version solves it on your site, so you only need this workaround if you can’t update yet.
You can disable the Assume that all texts in PHP strings are in English option on the WPMLTheme and plugin localization page.
Please note that this should resolve the issue in most, but not all, cases.