`clean_translations.py --strict` reported seven unused keys.
Three are genuinely dead: TrackSelectionHelper.getEmptyMessage was
removed as unreachable in 4307c49c, and the sheets that need an empty
state carry their own strings. Removes them from every locale.
The four accessibility keys are false positives. tv_number_spinner
aliased the subtree as `final a11y = ...accessibility`, which the static
scanner cannot follow — its docstring says as much. Binds the documented
`final t = Translations.of(context)` instead and hoists the two labels,
so the semantics stay identical and the scanner sees the chains.
Complete the contributed translations against the current English source and
register the locales in the language picker.
Fills the 51 keys the contributions predated, including the whole
downloads.backgroundWarning block that every locale must translate, and
restores the ${count} placeholder the sync-rule confirmation had dropped in all
three files.
Also corrects contributed strings: Azerbaijani "Imkan (Kopyalama)"
(possibility/copying) for resolution and several Turkish spellings, and Kazakh
Latin-script leaks ("Keyinirek", "Subtitr") plus Uzbek loanwords ("бош", "бир",
"муаммо") that do not read as Kazakh.
Consolidates #1689, #1690, and #1691.
Co-authored-by: Omc725 <98108290+Omc725@users.noreply.github.com>
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