You can use \n but you will have to provide some styling.
So in your template use this:
<div class="my-translated-paragraph">
{{'STRING_TO_TRANSLATE' | translate}}
</div>
Your en.json:
{
"STRING_TO_TRANSLATE": "text on first line.\nText on second line"
}
Your (s)CSS file:
.my-translated-paragraph{
white-space: pre-wrap;
}
More info an the magic behind white-space: https://stackoverflow.com/a/42354356/3757110
See also a github Issue about this: https://github.com/angular-translate/angular-translate/issues/595