How to concatenate strings in twig

This should work fine:

{{ 'http://' ~ app.request.host }}

To add a filter – like ‘trans’ – in the same tag use

{{ ('http://' ~ app.request.host) | trans }}

As Adam Elsodaney points out, you can also use string interpolation, this does require double quoted strings:

{{ "http://#{app.request.host}" }}