It would also be great to then allow different General Conditions depending on the language. I am now using the templates but something more reliable would be great.

This is very useful and goes beyond templates and customer portal language, it has todo with all terms that are translated based on the app's UI language. For example If I'm using English in my UI and I send a invoice template in Spanish, the payment term "Due on Receipt" should be translated into Spanish as well but it isn't since those terms are linked to the app's language.

Please do this for all modules!

Exactly, better overall multilingual usage would be really nice to have.

Mail templates could use translations and could be automatically used by the language set in the contact or by client in the portal. Default invoice template translations could be used by the client language as well.

I would add that it would be ideal to also have a drop down or a pre-defined language for a customer and the templates product descriptions are in that language. I have a company in Montreal and we are both French and English, hence the invoices need to be mailed or email in both language depending on who the client is. Is this possible??

Exactly what we want too – we have native English speakers and native Japanese speakers in the office. Each needs to be able to use Zoho Invoice in his/her own language (the interface) and raise invoices in English or Japanese according to the client. We're one (small) company so don't want to set up one 'English' organization and one 'Japanese' organization.