You can associate a language to a particular customer through price categories.
Price categories are a great tool because they also allow you to define several parameters such as accounting code or currency.
To create a price category, click Settings and Prices and discounts.
Add or edit a price category.
You can then choose the desired document layout.
To apply the price category to a client, open the file and click Edit.
In the window that opens, you can select the desired price category.