Hi Marc, Welcome to All Things Plants!
I hope someone in your part of the world will pop in with some advice but I'd think you could safely plant out in the garden after the last frost of the season. Tomatoes need warmth and light and many gardeners start seeds indoors in the cooler months and then transplant the seedlings into the garden once the weather warms up.
I don't know if this will help but when googling I found this blog from someone growing tomatoes in Sweden:
http://swedishtomatoes.blogspo...
http://swedishtomatoes.blogspo...
Again,
and happy gardening!
Lin
