Added localization strings for Slovak language

[metalllus]

I realized the payment sheet is not available for Slovak language (country: Slovakia, NOT Slovenia) on iOS. As I desperately need the implementation of the Slovak language for the Stripe payment sheet in my iOS app, I translated these strings into Slovak. I am a pro linguist with 15+ years of experience translating English into Slovak. I am not sure if this is sufficient to roll out Slovak language version on the payment sheet but I wanted to give it a try and show my willingness to help with pushing this forward. If you need any further assistance, do let me know.

[CLAassistant]

All committers have signed the CLA.

[csabol-stripe]

Thanks so much for the submission @metalllus! I will bring this to the attention of our localization team and figure out the best next steps :)