The language in Appinvoice can be changed in the account setting. To do this, click on the account icon in the upper right corner to open the account settings.
There you will find the field "Language". Choose your desired language and click save.
Currently available languages:
Set language for invoices
The language for invoices can be set independently of the user language. The language for each invoice can be changed directly during invoice creation. The language can also be changed after creation by opening/editing an existing invoice and setting the desired language.
At "Preferences->General" you can set a default language for the use of Appinvoice.