I found two major issues while building akka on Windows (with NetBeans):

1. UnsupportedCharsetException - this occurs because the locale charset (aka. codepage on Windows) cannot be set to utf-8 (MS-DOS heritage and bad architectural decisions)
2. Graphviz had to be installed and added to the PATH to succesfully compile scaladoc with diagrams

For the former issue, the resolution is only a single line change in AkkaBuild.scala, forcing to use utf-8 encoding:

diff --git a/project/AkkaBuild.scala b/project/AkkaBuild.scala
index dc69f4c..5aa1bf8 100644
--- a/project/AkkaBuild.scala
+++ b/project/AkkaBuild.scala
@@ -893,7 +893,7 @@ object AkkaBuild extends Build {
           val name = f.getName
           if (name.endsWith(".html") && !name.startsWith("index-") &&
             !(name.compare("index.html") == 0) && !(name.compare("package.html") == 0)) {
-            val source = scala.io.Source.fromFile(f)
+            val source = scala.io.Source.fromFile(f)("utf-8")
             val hd = source.getLines().exists(_.contains("<div class=\"toggleContainer block diagram-container\" id=\"inheritance-diner\">"))
             source.close()
             hd

Well, on the Eclipse based environment it might not occur because in Eclipse you can set the charset to utf-8 in the Preferences. But as the sbt is a console app and it can run stand alone, it's highly likely it will fail to compile on Windows.

If you find this fix worth to commit then I'll prepare a GitHub pull request along with.