Fix crash if no language is set
Fix crash if no language is set

If `cockpit.language` is undefined (which can happen in practice), just
default to English instead of crashing on calling
`undefined.includes()`.

jkozol/welder-web,weldr/welder-web,jkozol/welder-web,weldr/welder-web,jkozol/welder-web,weldr/welder-web,jkozol/welder-web,weldr/welder-web