Read the lines into a list and return the value index-1
Depends on the fact the list is not too big and it reads the hole file into a list.
Alternatively you can use Stringreader to read lines until you reach the wanted index. Probably much slower then using a List.
B4X:
Log(GetLine(1))
Log(GetLine(10))
Sub GetLine(idx As Int) As String
Dim text As List = File.ReadList(File.DirAssets,"test.txt")
If text.IsInitialized And text.Size>=idx-1 Then
Return text.Get(idx-1)
End If
Return ""
End Sub