for whatever reason, setting the content type using okhttputils2 is a special case:
USE: j.GetRequest.SetContentType("application/json")
NOT: j.GetRequest.SetHeader("Content-Type", "application/json")
and although at guick glance your json looks ok, you should learn to create json strings in the b4x manner:
Dim json As JSONGenerator
Dim postmap As Map
postmap.Initialize
Dim phrases As List = Array As String("Hello,World!")
postmap.Put("text", phrases)
postmap.Put("target_lang", "DE")
json.Initialize(postmap)
log(json.tostring) ' {"target_lang":"DE","text":["Hello,World!"]}