Update translations

[davidxuang]

• Add translations for two Chinese variants and correct their language code according to official modding manual (also done for Japanese);
• Fix some typo in English texts.

[Vacuum-Tube]

Tipp for multi line strings: You can avoid the "\n" .. stuff with the following syntax:

return {
    en = {
        mod_name = "My mod",
        mod_desc = [[
This is a description.
The whole block is recognized as text, making it easier to deal with line breaks.
...
]],
        translation_string = "My translation",  -- this is the same like below
        ["translation_string"] = "My translation",
        ...