I want packing slip always in the customers language

I’m not related to (and don’t use) this plugin. But a guess is that if admin is running the script to generate this, then, obviously, admin′s chosen language will be used. In other words, you’d need to temporarily change your admin language to the needed target language. (Or somehow track the chosen language of the customer, which may not be too easily done, if the customers don’t make a full registration on the site…)

That’s the workaround I am using today, but it would be nice if there would be a setting for this functionallity.

Our plugin follows WooCommerce conventions when it comes to languages used around order information:

• When an email is sent to the customer, this is always in the customer language, regardless of the active backend language – so is the attached PDF
• When a PDF is created in the backend, it is in the current active language

Actively overriding this can cause issues of invoices being only partly translated (since most other plugins also follow this convention).
For multilingual setups like WPML or Polylang, we have the Professional extension that integrates with their language switchers: this defaults to the customer language but also allows to always use a single language for all customers (by a setting).
Multilingual WordPress is quite complex and we prefer sticking to conventions to make this work equally well for everyone.

If you need to print the PDF in a specific language, then temporarily switching the language as @tobifjellner suggested is the way to go.

I understand how you are thinking, thanks for the swift reply.