Allow set invoice language

[Nisgrak]

Hi!

I see this app and seems like it's what I need.

But I need to change the strings to other language (Spanish), I would love to make a PR with the correct strings if you make a system to change the language.

Thanks for your work.

[relistan]

@Nisgrak Sorry I'm so far behind on responding. It's a good idea and I appreciate the offer. Let's see if I can make any time to do that.