Part of the challenge will be that this is set up as Directive at European level, so is then implemented independently by member states through their own local regulations.
I certainly think it would be acceptable in the UK, as whilst there are specific criteria for both "Natural Mineral Water" and "Spring Water", for "Water" it would generally be acceptable to use several sources, subject to the usual requirement to not mislead the consumer. Nonetheless it's an unusual question as normally it is the other way around - most brands want to actively promote/advertise that they're using spring water!
I'm not sufficiently familiar with Slovakian legislation to be able to confidently advise on that specifically though, so you may need to check with your local regulatory body (or the regulator in the countries of sale, if different).