Does Craft save dates in the locale format?
Where are you seeing the incorrect date, in the database or when you output it in the templates?

I think Craft is probably working as expected here.

Craft will expect the date submitted to be in the correct format for the locale it was submitted from, so will translate it from that format into UTC to save in the database.

Then, when the date is output in templates, it is transformed again to the correct format for the locale it is being displayed in.

The EN locale presumably assumes US date formatting so 01/02/2018 is correct as 2nd January 2018. For European locales, a date submitted as 01/02/2018 will correctly be recognised as 2nd February 2018.

You need to make sure that the date input in your form is formatted according to the expected way for the locale the form is in.