In what language should terms and agreements be written?

I'm launching SaaS which will be accessed mainly through mobile app and web browser. Company will be located in one of the EU countries, but will offer services worldwide.

I'm wondering in what language should I write terms and agreements? Should I use local language or should it be in english? And also to what laws should I refer to?

P.S. Mobile app and website will be multilingual.

The original version of your terms and conditions should be in the language of the country where your company is located or in your customer's language. As long as the terms and conditions are not an employment contract, you and your customers are free to chose the language you think suits you best, but your customers must first agree to sign terms and conditions in other language than yours or theirs.

In B2C relations within the EU you most likely should refer to laws of the country where your company is incorporated. You must comply with the local VAT rules and data protection acts. You should contact your local information commissioner and check whether you have to register any of your databases.

If you sell services outside the EU, you should register for VAT in each of the countries you sell the services in.

if your site english, use english for your terms. it may look weird if your website is english and your term is deutsch(german)

if you plan to use a multi-language for your website, that would be better

It depends I think on the language you choose as a main language for your service. I see it will be multilingual, so I still think - prepare english version and then translate it if possible to other languages, but if there is no translation leave it in english (and translate later as you go).

And about the laws, I think that you should refer to the ones where your company or activity is registered. But I'm not the best lawyer in the world :D