feat(app): a studyset's languages, and Chinese and Korean fonts

The studyset sheet gains Languages, as before: off until "Set
languages", then the language of the cards' fronts and of their backs
(22 languages, Chinese in its two scripts). A studyset without its own
takes those of the nearest studyset above it, both together; set for the
first time, they start from those it took. The payload held both fields
already.

They set its text: Noto Sans SC, TC and KR join Noto Sans JP (bundled,
never fetched; 34 MB more, fonts 42 MB in all, your call), and since
Han characters take the shapes of the first font that has them, every
screen inside a studyset (its screen, cards, notes, paths, studying it)
puts the font of its front's language first, else of its back's,
Japanese otherwise. Every studyset route goes through one helper now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -307,6 +307,35 @@
   "studysetSheetDescriptionAdd": "Tippen, um eine Beschreibung hinzuzufügen",
   "studysetSheetDescriptionAdded": "Beschreibung hinzugefügt — zum Bearbeiten tippen",
   "studysetSheetInheritColor": "Erben",
+  "studysetSheetLanguages": "Sprachen",
+  "studysetSheetSetLanguages": "Sprachen festlegen",
+  "studysetSheetLanguagesHelper": "In welcher Sprache die Seiten der Karten sind. Chinesisch, Japanisch und Koreanisch erscheinen dann in ihren eigenen Schriftzeichen.",
+  "studysetSheetLanguagesInherited": "Ausgeschaltet übernimmt es die Sprachen des Lernsets, in dem es liegt.",
+  "studysetSheetFrontLanguage": "Sprache der Vorderseite",
+  "studysetSheetBackLanguage": "Sprache der Rückseite",
+  "studysetSheetLanguageNotSet": "Nicht festgelegt",
+  "languageArabic": "Arabisch",
+  "languageGerman": "Deutsch",
+  "languageGreek": "Griechisch",
+  "languageEnglish": "Englisch",
+  "languageSpanish": "Spanisch",
+  "languageFrench": "Französisch",
+  "languageHebrew": "Hebräisch",
+  "languageHindi": "Hindi",
+  "languageItalian": "Italienisch",
+  "languageJapanese": "Japanisch",
+  "languageKorean": "Koreanisch",
+  "languageDutch": "Niederländisch",
+  "languagePolish": "Polnisch",
+  "languagePortuguese": "Portugiesisch",
+  "languageRussian": "Russisch",
+  "languageSwedish": "Schwedisch",
+  "languageThai": "Thailändisch",
+  "languageTurkish": "Türkisch",
+  "languageUkrainian": "Ukrainisch",
+  "languageVietnamese": "Vietnamesisch",
+  "languageChineseSimplified": "Chinesisch (vereinfacht)",
+  "languageChineseTraditional": "Chinesisch (traditionell)",
   "fieldRequired": "Pflichtfeld",
 
   "exploreTitle": "Entdecken",