Using two languages will give you a much better cost/benefit ratio, and here's why. In a team of 20 people, 18 will program in simple language A, while 2 will program in simple language B. The ones programming A know it well; those programming B know it well, and usually they would also know A well, as they would probably be your star programmers. The JVM makes integrating the two quite easy. You wouldn't need to discipline those 18 programmers to limit themselves to some subset of Scala, and they wouldn't be dumbfounded by its linked-list.
I've worked on projects that used C++, Ada and FORTRAN; on projects that used Java, C, and some homegrown DSL; on projects that used C++ and Scheme. In all circumstances, the integration of languages proved simpler than "disciplining" the C++ programmers, and on the JVM that integration is a lot simpler than what we had to put up with back then.
Also, bear in mind that team members change, as do team leaders. That proved to be a big problem for C++ projects as the discipline changed over the lifetime of the project.
First, Scala is not C++. Second, you need good discipline in those simple languages, as well. I've seen too much terrible Java code to believe Java simplicity saves you from being disciplined.
I've worked on projects that used C++, Ada and FORTRAN; on projects that used Java, C, and some homegrown DSL; on projects that used C++ and Scheme. In all circumstances, the integration of languages proved simpler than "disciplining" the C++ programmers, and on the JVM that integration is a lot simpler than what we had to put up with back then.
Also, bear in mind that team members change, as do team leaders. That proved to be a big problem for C++ projects as the discipline changed over the lifetime of the project.