feat(localization): add difficult vocabulary marking features in multiple languages

- Introduced new localization strings for marking vocabulary as difficult, including options to save and remove difficult marks.
- Enhanced user experience by providing feedback on the status of difficult vocabulary in Cebuano, Spanish, and French.
- Updated existing localization files to ensure consistency across languages, supporting the recent vocabulary management features.

frontend/src/i18n/locales/fr/socialnetwork.json

@@ -457,7 +457,10 @@
                 "askedVocab": "Demandé :",
                 "stats": "statistiques",
                 "success": "Succès",
-                "fail": "échec"
+                "fail": "échec",
+                "hardCount": "Marqués difficiles",
+                "markHard": "Marquer comme difficile",
+                "unmarkHard": "Retirer le marquage difficile"
             },
             "search": {
                 "open": "Recherche",
@@ -809,6 +812,13 @@
                 "trainerProgressNewContent": "Nouveau contenu : {current}/{target}",
                 "trainerProgressReview": "Répéter : {count}",
                 "trainerProgressMixShare": "Proportion de mélange : {pourcentage} %",
+                "markVocabHard": "Mark as difficult",
+                "markVocabHardSaved": "Vocabulary marked as difficult.",
+                "unmarkVocabHard": "Remove from difficult list",
+                "unmarkVocabHardSaved": "Vocabulary removed from difficult list.",
+                "hardVocabModeActive": "Intensive block: difficult vocabulary",
+                "hardVocabRemaining": "Remaining until stable: {count}",
+                "startHardVocabTrainer": "Train difficult vocabulary ({count})",
                 "unknownExerciseTypeNotice": "Ce type d'exercice n'est pas encore affiché de manière interactive dans la vue actuelle.",
                 "unknownExerciseTypeLabel": "Tapez : {type}",
                 "lessonReviewHeadlineDone": "Cette leçon a atteint le niveau d'immersion libre.",