Werk #7086: Localizations now extend shipped localizations

Locally installed localization files (local/share/check_mk/locale) may now extend the builtin localizations instead of overriding them.

In previous versions the local locale file had to contain all texts that we ship with our standard localizations, for example the german localization in case one wanted to extend / change a single text.

Now both, the builtin and local localization files are loaded. All texts are searched in the local file and looked up in the builtin file as fallback in case the text can not be found in the local file.

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