Update Czech translation for the new Stretchly

[bramborman]

Issue: #584

Requirements

• issue was opened to discuss proposed changes before starting implementation.
• during development, node version specified in package.json was used (ie using nvm).
• package versions and package-lock.json were not changed (npm install --no-save).
• app version number was not changed.
• all new code has tests to ensure against regressions.
• npm run lint reports no offenses.
• npm run test is error-free.
• README and CHANGELOG were updated accordingly.

Description of the Change

I updated the Czech translation. I copied the Slovak strings and based new strings on them.

I also found a few typos in the Slovak translation so I fixed them too (at least I hope I got them right as it's not my native language 😃 If not, feel free to correct me 😄 ).

Other information

I don't have node installed so I didn't test the changes but I think there should be no issues.

[hovancik]

Thanks, I love how you fix my Slovak :D