To your assets/i18n/en.json:
"loggedin_welcome_title": "<strong>Welcome</strong> {{username}}, to your kiosk."
To your assets/i18n/nl.json:
"loggedin_welcome_title": "<strong>Welkom</strong> {{username}}, bij jouw kiosk."
Then in your page.html.twig you can use:
<h1 class="mt-0" [innerHTML]="'loggedin_welcome_title'|translate:{'username': currentUser.name }"></h1>
Another example is:
"show_x_results": "Show {{number}} results"
And then have the following in your templates:
<strong>{{ 'show_x_results'|translate: {'number': totalResultsControl.value} }}</strong>
To make this work, make sure you have your ngx-translate pipe working!