Hi,
That is the way or Polylang string translations. Unfortunately .mo .po method only works on strings that are statically hard-coded. Since these texts are changeable, the only way to do it is via a string translator module – in this case WPML and Polylang are supported. Unfortunately as far as I know Loco translate only works with static content.
I wish there was a way to somehow make it work via the .pot translating system, but does not work with dynamic string by design.