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/es/socialnetwork.json

@@ -457,7 +457,10 @@
                 "askedVocab": "Preguntado:",
                 "stats": "Estadísticas",
                 "success": "Éxito",
-                "fail": "Fallo"
+                "fail": "Fallo",
+                "hardCount": "Marcadas como difíciles",
+                "markHard": "Marcar como difícil",
+                "unmarkHard": "Quitar marca de difícil"
             },
             "search": {
                 "open": "Buscar",
@@ -809,6 +812,13 @@
                 "trainerProgressNewContent": "Contenido nuevo: {current}/{target}",
                 "trainerProgressReview": "Repaso: {count}",
                 "trainerProgressMixShare": "Parte mezclada: {percent}%",
+                "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": "Este tipo de ejercicio todavía no se muestra de forma interactiva en la vista actual.",
                 "unknownExerciseTypeLabel": "Tipo: {type}",
                 "lessonReviewHeadlineDone": "Esta lección ya ha llegado a la fase de práctica libre.",