Add czech translation

[breadcorp]

heya, ive noticed there is no czech translation, and this repo looks great for learning

im down to help with translations :D

[ledsun]

Hi, thanks for offering to help! A Czech translation would be very welcome.

Please take a look at our localization guide and use the existing translations under i18n/ and problems/ as examples.
The language code for Czech should be cs.

The translation would mainly include:

• i18n/cs.json, based on menu.json
• i18n/footer/cs.md
• i18n/troubleshooting_cs.md
• problem_cs.md and solution_cs.md for each exercise

Please run npm test and also launch javascripting to check the translation from the CHOOSE LANGUAGE menu. Feel free to open a draft PR early if you would like feedback while working on it.

Thanks again!