Czech language translation

[digitaltrails]

Add support for the Czech language. (See #30 for background info.)

[digitaltrails]

I've added cs_CZ.ts and about_cs_CZ.txt

The initial cs_CZ.ts translation was generated by Google Translate. Deepl seemed to be struggling with some of the technical terms, but if google translate has made a mess of it, I can make another attempt using Deepl and then overlay any it missed with google's values.

What needs doing:

1. Find any translations in cs_CZ.ts marked unfinished and see if they actually need a translation.
2. Check that the cs_CZ.ts automatically generated translations are correct/appropriate and correct any that need attention.
3. Check about_cs_CZ.txt similarly.
4. The copyright and legal notice in about_cs_CZ.txt has to remain in English so that the legal interpretation is unambiguous.
5. Check any @amp; values in the .ts file - these defined the Alt-Key shortcuts, the shortcuts need to be unique (I think there might be two O-shortcuts in the generated translations).
6. Add notes or issues if anything isn't able to be translated because I forgot to allow for it in the code.

Testing:

Various testing options that can be combined as necessary:

Preliminary setting change:

In the vdu_controls Settings Dialog enable translations.

Testing with a desktop that is already set to the target LANG

1. Put the translation files cs_CZ.ts and about_cs_CZ.txt into your desktop's ~/.config/vdu_controls/translations/ folder.
2. Check that LANG and LC_ALL and LANGUAGE environment variables are correctly set. What is required here might vary from one Linux distribution to another.
3. Start a new vdu_controls.

Testing without setting the whole desktop to LANG

To temporarily test without setting the LANG/LC-ALL/LANGUAGE for the entire desktop, run vdu_controls from the command line:

% LANG=da_DK VDU_CONTROLS_DEVELOPER=yes LC_ALL=cs_CZ LANGUAGE=cs_CZ python3  vdu_controls.py

Testing without having to copy the translation files out of the source folder

If the environment variable VDU_CONTROLS_DEVELOPER=yes then vdu_controls will additionally look for a translation folder in the current working directory. This is useful when working in the source folder:

% cd github-clone-folder/vdu_controls/   # Cloned copy of project
% ls translations/                       # Check translations are actually present
% export VDU_CONTROLS_DEVELOPER=yes      # Trigger developer options
% LANG=da_DK VDU_CONTROLS_DEVELOPER=yes LC_ALL=cs_CZ LANGUAGE=cs_CZ python3  vdu_controls.py

I think that covers it.