Source

File: wp-includes/class-wp-editor.php

public static function wp_mce_translation( $mce_locale = '', $json_only = false ) {
		if ( ! $mce_locale ) {
			$mce_locale = self::get_mce_locale();
		}

		$mce_translation = self::get_translation();

		foreach ( $mce_translation as $name => $value ) {
			if ( is_array( $value ) ) {
				$mce_translation[ $name ] = $value[0];
			}
		}

		/**
		 * Filters translated strings prepared for TinyMCE.
		 *
		 * @since 3.9.0
		 *
		 * @param array  $mce_translation Key/value pairs of strings.
		 * @param string $mce_locale      Locale.
		 */
		$mce_translation = apply_filters( 'wp_mce_translation', $mce_translation, $mce_locale );

		foreach ( $mce_translation as $key => $value ) {
			// Remove strings that are not translated.
			if ( $key === $value ) {
				unset( $mce_translation[ $key ] );
				continue;
			}

			if ( false !== strpos( $value, '&' ) ) {
				$mce_translation[ $key ] = html_entity_decode( $value, ENT_QUOTES, 'UTF-8' );
			}
		}

		// Set direction.
		if ( is_rtl() ) {
			$mce_translation['_dir'] = 'rtl';
		}

		if ( $json_only ) {
			return wp_json_encode( $mce_translation );
		}

		$baseurl = self::get_baseurl();

		return "tinymce.addI18n( '$mce_locale', " . wp_json_encode( $mce_translation ) . ");\n" .
			"tinymce.ScriptLoader.markDone( '$baseurl/langs/$mce_locale.js' );\n";
	}