I like Invoice Plane , and would like to share how the multilanguage is done in my previous paid system .
All invoices are always using system language , however near the action buttons (SEND, PRINT, DOWNLOAD) there is s dropdown with availiable translations . Thus when you send to client - you pick one language and click send. But for yourself you get it in default language.
This is quite easy to use .
At the moment , i know i can set clients language , and invoice will be in secondary language , BUT when i need sending invoice to accountancy - i need to do this in native primary language , thus i have to change language on CLIENT tab to get it changed.
Hopefully DEVs will notice this post