put new line in string to translate

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