[WordPress] Exclude text domains from language files by wrapping them into custom functions instead of default l10n functions. (http://glck.be/6651/)

wcl10n-example.php

<?php
/* String from your theme/plugin */
_e( 'Margaritas!', 'your-textdomain' );
__( 'Margaritas!', 'your-textdomain' );

/* Original string from WooCommerce: */
_e( 'Product', 'woocommerce' );
__( 'Product', 'woocommerce' );

wcl10n-replacement.php

<?php
/* In your own theme/plugin replace with: */
wcl10n_e( 'Product', 'woocommerce' );
wcl10n__( 'Product', 'woocommerce' );

wcl10n.php

<?php 
/**
 * Wrappers for default l10n functions to include in your theme or plugin.
 */

if ( ! function_exists( 'wcl10n__' ) ) {
	/**
	 * Wrapper for __() gettext function.
	 * @param  string $string     Translatable text string
	 * @param  string $textdomain Text domain, default: woocommerce
	 * @return void
	 */
	function wcl10n__( $string, $textdomain = 'woocommerce' ) {
		return __( $string, $textdomain );
	}
}

if ( ! function_exists( 'wcl10n_e' ) ) {
	/**
	 * Wrapper for _e() gettext function.
	 * @param  string $string     Translatable text string
	 * @param  string $textdomain Text domain, default: woocommerce
	 * @return void
	 */
	function wcl10n_e( $string, $textdomain = 'woocommerce' ) {
		echo __( $string, $textdomain );
	}
}

/* repeat for other l10n function, see wp-includes/l10n.php */

The _e function in WordPress echoes the string, so you can't return it. This means your second example here doesn't quite work as expected. Use echo __( $string, $textdomain ) instead.

Bummer, thanks. 👍