Adding support for et-EE, hr-HR, lv-LV and sk-SK

[diogodadalt]

This PR adds support for the following languages:

• et-EE
• hr-HR
• lv-LV
• sk-SK

unicode.org was used as a reference for their implementation:

• https://www.unicode.org/cldr/cldr-aux/charts/28/verify/dates/et.html
• https://www.unicode.org/cldr/cldr-aux/charts/28/verify/dates/hr.html
• https://www.unicode.org/cldr/cldr-aux/charts/28/verify/dates/lv.html
• https://www.unicode.org/cldr/cldr-aux/charts/28/verify/dates/sk.html