Add i18n files and set first translations

docs/i18n_languages.md

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+# I18N Guidelines
+
+In JSON file:
+
+- Keep the file sorted
+- Always write in lowercase
+
+In GUI:
+
+- If the first letter of the translation must be uppercase, use the postProcess, for example: `{t_auth('advanced_users', { postProcess: 'capitalize' })}`