I have two translatable strings as follows in my 404.php file
esc_html_e( 'I couldn't find the page you were looking for.', 'themename' ); esc_html_e( 'Try a search or one of the links below.', 'themename' );
At its present form, the first string does not get translated. Escaping the single quote with a backslash does not help either.