The Developer’s Guide to Full WordPress Plugin Localization (Including JavaScript & Calendars)
in For Developers, Plugin, Wordpress on December 1, 2025How to Properly Localize a WordPress Plugin (Without Breaking the Calendar)
If you’ve ever tried to translate a WordPress plugin, you’ve probably run into this frustrating scenario: 90% of your plugin is in Italian (or any language), but the calendar widget, datepicker, or confirmation form stays stubbornly in English.
You’re not doing anything wrong — you just missed a few critical details that most localization guides skip.
In this guide, we’ll walk through the complete, battle-tested workflow to fully localize your WordPress plugin — including JavaScript-powered components like calendars — so your users get a seamless, consistent experience in their language.
✅ Step 1: Folder Structure & File Naming
Start with a clean, standard structure inside your plugin:
your-plugin/
├── your-plugin.php
├── …
└── languages/
├── your-plugin.pot
├── your-plugin-it_IT.po
└── your-plugin-it_IT.mo
Important: The textdomain in your code must match your plugin’s folder name (or the slug you choose). If your plugin folder is
appointment-management-pro-sws, your textdomain should beappointment-management-pro-sws— notwp-appointment-management-sws(unless that’s your actual folder name). Consistency here prevents silent failures.
✅ Step 2: Load the Text Domain Correctly
In your main plugin file (your-plugin.php), hook into init (not plugins_loaded) to load translations after WordPress has set the user’s locale:
function your_plugin_load_textdomain() {
load_plugin_textdomain(
'your-plugin', // ← must match .po/.mo file prefix
false,
dirname(plugin_basename(__FILE__)) . '/languages/'
);
}
add_action('init', 'your_plugin_load_textdomain');
Why
init? Because some multilingual plugins (like WPML or Polylang) set the locale duringplugins_loaded. Waiting untilinitensures you get the right language.
⚠️ Step 3: The Hidden Problem — JavaScript & Calendars
Here’s where most plugins fail: PHP strings translate fine, but JavaScript elements (like jQuery UI Datepicker, Flatpickr, or custom booking calendars) stay in English.
Why? Because JavaScript runs in the browser — it doesn’t know about your .mo files.
✅ Solution: Localize JavaScript with wp_localize_script
- Enqueue your JS file (e.g.,
booking.js). - Pass translated strings from PHP to JS:
// In your admin or frontend script loader
wp_enqueue_script('your-plugin-booking', plugin_dir_url(__FILE__) . 'js/booking.js', ['jquery'], '1.0', true);
$translated_strings = [
'nextMonth' => __('Next month', 'your-plugin'),
'prevMonth' => __('Previous month', 'your-plugin'),
'weekdays' => [
__('Sun', 'your-plugin'),
__('Mon', 'your-plugin'),
// ... etc.
],
'confirmText' => __('Confirm your appointment', 'your-plugin'),
];
wp_localize_script('your-plugin-booking', 'YourPluginI18n', $translated_strings); - Use those strings in your JavaScript:
// booking.js
console.log(YourPluginI18n.confirmText); // Now in the user's language!
// If using Flatpickr:
flatpickr("#datepicker", {
locale: {
firstDayOfWeek: 1,
weekdays: {
shorthand: YourPluginI18n.weekdays,
longhand: [
__("Sunday", "your-plugin"),
// ... (or pass full array from PHP)
],
},
months: {
shorthand: [
__("Jan", "your-plugin"),
// ...
],
longhand: [
__("January", "your-plugin"),
// ...
],
},
}
}); Pro Tip: For libraries like FullCalendar or Datepicker, check if they already include locale files. If so, enqueue the right one (e.g., jquery-ui/i18n/datepicker-it.js) based on get_locale().
✅ Checklist: 5 Things Developers Forget When Localizing Plugins
Mismatched textdomain → .po filename ≠ textdomain in code → silent failure.
Hardcoded strings → Always wrap user-facing text in __() or _e().
Escaping without localization → Use esc_html__() and esc_attr_e() for safe, translatable output.
Missing JavaScript translations → The #1 reason calendars/forms stay in English.
Not testing in a real multilingual environment → Install Loco Translate or switch WPLANG to verify everything renders correctly.
Final Thought
Full localization isn’t just about .po files — it’s about respecting your users’ language in every layer of your plugin, from PHP to JavaScript.
When your calendar finally displays “Gennaio” instead of “January” for an Italian user, you’re not just translating words — you’re delivering a professional, trustworthy experience.
And that’s what turns free users into loyal customers.