[l10n][pickers] Add Slovak (sk-SK) locale

[MatejFacko]

Add Slovak (sk-SK) locale for date-pickers, improve (cz-CZ) locale

[mui-bot]

Localization writing tips ✍️

Seems you are updating localization 🌍 files.

Thank you for contributing to the localization! 🎉 To make your PR perfect, here is a list of elements to check: ✔️

• Verify if the PR title respects the release format. Here are two examples (depending if you update or add a locale file)

  [l10n] Improve Swedish (sv-SE) locale
  [l10n] Add Danish (da-DK) locale

• Update the documentation of supported locales by running yarn l10n
• Verify that you have added an export line in src/locales/index.ts for the new locale.
• Run yarn docs:api which should add your new translation to the list of exported interfaces.
• Clean files with yarn prettier.

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Updated pages

No updates.

These are the results for the performance tests:

Test case	Unit	Min	Max	Median	Mean	σ
Filter 100k rows	ms	597	887.1	634	718.14	131.691
Sort 100k rows	ms	597.4	1,063.4	865.8	863.34	150.639
Select 100k rows	ms	216.6	312.7	239.3	254.92	38.288
Deselect 100k rows	ms	154.5	311	180.6	200.72	57.526

Generated by 🚫 dangerJS against a0acb9c

[alexfauquette]

Nice PR 👌

Thanks :)