TYPO3 Core

This is something to be tackled within TYPO3 Core, not within "translations". Alternatively, if the text is only "wrong" in German (as in your screenshot) and not in the main language (English), then you could suggest a new translation right away within the corresponding project on https://translation.typo3.org.

You may want/need to enable debugging to figure out which label is being used: http://xavier.perseguers.ch/tutoriels/typo3/articles/managing-localization-files.html#c950

$GLOBALS['TYPO3_CONF_VARS']['BE']['lang']['debug'] = true;

I moved the bug the Core project. Instead of the label "labels.hidden" (typo3_src/typo3/sysext/lang/locallang_core.xlf) there should be used "disabled". Have a look at the attached screenshot (en_translate.PNG)

Does not happens only to "pages" in pagetree, but also in list module and, in general, for every content element.

We could decide that "hidden" must be changed with "disabled" everywhere, then.

But what happens to the "switch" "Un-hide/Hide record"? (See attached screenshot) - Should it be Enable/Disable record ?

Even this one is confusing most people are actually used to this,
so i would not like to see changes here except for master
- otherwise we will create even more confusion with the bugfix releases.

I agree on that we should unify this across all records and in my oppinion a page is not really different fom any other record.
That said i would propose that change the wording across all record types to disabled, since this is actually more reflecting
the actual behavior than hidden is currently doing.

Also the tca pointer that maps to hidden in tt_content is named disabled.
https://docs.typo3.org/typo3cms/TCAReference/Ctrl/Index.html#enablecolumns

Hidden:
to conceal from sight; prevent from being seen or discovered:

Disabled:
having become or having been rendered inoperative:

On current master I can see that translation of this element is changed to "Page visible" and for content elements it's "Visibility of content element", so I think this issue is no longer valid? Looks like naming is unified right now.