For developers

You don’t need this page to use LingoPress

This page is for whoever builds or maintains your site. Everything LingoPress does is set up from the dashboard, as explained in the other guides. Start with First steps.

Conventions: public functions and hooks use the lingopress_ prefix, the namespace is BallesDev\LingoPress and the text domain is ballesdev-lingopress. No Composer and no external dependencies.

Public API (PHP)

Languages

Function	What it does
lingopress_languages_list( $args )	List of the site’s languages.
lingopress_default_language( $field )	Default language (slug or another field).
lingopress_current_language( $field )	Language of the current request (frontend).
lingopress_add_language( $args )	Creates a language (rewrite rules are flushed automatically).

Content

Function	What it does
lingopress_set_post_language( $post_id, $lang )	Assigns a language to a post.
lingopress_get_post_language( $post_id, $field )	Language of a post.
lingopress_save_post_translations( $translations )	Links posts as translations ([ 'es' => 12, 'en' => 34 ]).
lingopress_get_post_translations( $post_id )	Translation group of a post.
lingopress_get_post( $post_id, $lang )	ID of a post’s translation in a given language.
lingopress_set_term_language / lingopress_get_term_language	Same, for terms.
lingopress_save_term_translations / lingopress_get_term_translations / lingopress_get_term	Same, for terms.

Posts created before LingoPress was activated have no language until they’re saved again; lingopress_set_post_language() assigns it in bulk from a script.

Display and strings

Function	What it does
lingopress_the_languages( $args )	Renders the language switcher. Same options as the shortcode (see Language switcher and menus).
lingopress_register_string( $name, $text )	Registers a translatable string in String translations.
lingopress__( $text ) / lingopress_e( $text )	Returns / prints the translation of a registered string in the current language.
lingopress_translate_string( $text, $lang )	Translation of a string into a specific language.

Register strings on every request, admin included: String translations lists the strings registered while it loads, and Remove unused strings deletes the saved translations of strings that weren’t registered on that request.

Hooks

Hook	Type	When
lingopress_language_defined	action	The request’s language has been resolved (frontend, in setup_theme).
lingopress_translated_post_types	filter	List of translatable post types (posts and pages by default; attachment when media translation is on).
lingopress_translated_taxonomies	filter	List of translatable taxonomies (category and post_tag by default).
lingopress_ai_system_prompt	filter	Lets you modify the system prompt sent to the AI.
lingopress_ai_provider	filter	Lets you replace the AI provider (inject your own).
lingopress_switcher_items	filter	Modifies the items in the language switcher.
lingopress_sync_post_meta	filter	Adds your own metadata to the sync between translations.
lingopress_hreflang	filter	Map of hreflang code => URL before it’s printed.
lingopress_canonical_redirect	filter	Canonical URL before redirecting; return false or '' to cancel the redirect.

Other types of content

Shop products, events and other custom post types or taxonomies aren’t translatable out of the box. Add them with the filters:

add_filter( 'lingopress_translated_post_types', function ( $post_types ) {
  $post_types[] = 'product';
  return $post_types;
} );

add_filter( 'lingopress_translated_taxonomies', function ( $taxonomies ) {
  $taxonomies[] = 'product_cat';
  return $taxonomies;
} );

The AI translates the title, content and excerpt; custom fields (post meta) are neither translated nor copied, unless you add them to the sync with lingopress_sync_post_meta.

REST API

• lang fields (read/write, with capability checks) and translations fields in the wp/v2/* collections of the translatable post types and taxonomies.
• Filter collections by language with ?lang=: GET /wp-json/wp/v2/posts?lang=en. lang=all turns the filter off.
• Custom endpoints: GET /wp-json/lingopress/v1/languages (public); POST and DELETE require manage_options.
• The block editor preloads its collections already filtered by the language of the post being edited.

WP-CLI

• wp lingopress import-polylang (since 1.1.1): imports languages, content languages and translation groups from Polylang, whether it’s active or not. Safe to run more than once: anything already in LingoPress is skipped, and the default language set in LingoPress is kept. Exits with an error when the site has no Polylang data.

API keys in wp-config.php

Keys defined as constants take priority over the ones saved in LingoPress → AI translation, and the screen shows them as “Defined via” the constant:

define( 'LINGOPRESS_ANTHROPIC_API_KEY', '...' );
define( 'LINGOPRESS_GEMINI_API_KEY', '...' );
define( 'LINGOPRESS_OPENAI_API_KEY', '...' );
define( 'LINGOPRESS_GROQ_API_KEY', '...' );

With the WordPress (Connectors) provider (WordPress 7.0+), LingoPress uses wp_ai_client_prompt() and stores no key.

Data model

Four custom taxonomies, no custom tables: lingopress_language and lingopress_term_language (language of posts and terms), lingopress_post_translations and lingopress_term_translations (translation groups). The same model proven by Polylang, with its own prefix: it survives WXR exports/imports and can coexist with Polylang’s data during a migration.