Add Turkish language/translations

Stalwart has i18n for calendar invite mails and such. Having the Turkish(tr) language would be very nice.

I have a branch in my fork: GitHub - yusufkandemir/stalwart at i18n/turkish · GitHub

If the maintainers are willing to accept a PR and vouch for me, I can create one. I haven’t created it yet as per CONTRIBUTING.md

We also need Turkish support. Please consider adding this soon.