Add localization keys for fallback and status messages in UI

Replaced hardcoded status and fallback messages with localized strings across downloader, update, and main UI modules. Updated language files for all supported languages to include new keys for fallback download status, pip timeout, invalid URL, zero selected, and playlist selection dialog title. This improves internationalization and consistency of user-facing messages.

src/gui/ytsage_gui_dialogs/ytsage_dialogs_selection.py

@@ -198,7 +198,7 @@ class SubtitleSelectionDialog(QDialog):
 class PlaylistSelectionDialog(QDialog):
     def __init__(self, playlist_entries, previously_selected_string, parent=None) -> None:
         super().__init__(parent)
-        self.setWindowTitle("Select Playlist Videos")
+        self.setWindowTitle(_("playlist.select_videos_title"))
         self.setMinimumWidth(500)
         self.setMinimumHeight(400)  # Allow more vertical space