Hello Marta Bartnicka ,

This is a question that is being raised more and more often and is addressed in our SDL Privacy Policy. 

https://www.sdl.com/about/privacy/sdl-language-cloud-privacy-policy.html

3. How SDL plc may use the source material 

• To provide the services ordered: SDL represents that during the machine translation process none of the source material will be needed, stored or used in SDL's translation engines, except for term dictionaries, which will be stored securely. 

SDL do not use any of your source material and our engines are trained only with publicly available content.

Many clients may allow you to use our SDL MT services once it is understood how we process this information.

Thank you very much, this answers my question (and can be presented to clients).