After installing the app, all your service desk projects will be prepared to use it, but the translation will initially be disabled.
You can use the "Get started" button in the app entry in the "Manage apps" section in Jira
or use the "Language Translation - Jira Service Desk" link in the left navigation panel in the "Manage apps" section.
Alternatively you can also select any service desk project, go to project settings and there click on "Language Translation".
In that case you can jump to chapter two below.