Make GNOME templates translation friendly

Affected version

• Nightly flatpak: Cannot test it due to template-related issues.
• Other: 46.1

Steps to reproduce

1. Create a new project via the 'Create a New Project' button.
2. Select the Vala language.
3. Choose GNOME or GTK template.

Current behavior

Both templates do not properly support gettext translation. Developers should modify some code to ensure proper translatability of the app.

Expected behavior

Ensure that the GNOME and GTK templates are properly translatable with minimal changes, similar to the C and Python language templates, which better support translatable apps.

Additional information

Other GNOME templates might have the same issue and need to be checked.