I have a nasty feeling you equate knowledge of a particular language with software development expertise. A decent software developer should be able to pick up a new language over a long weekend - these aren't spoken languages we're talking about here, these are human-designed artefacts with a set (and generally very small) syntax and a bunch of libraries. Learning, or indeed making, a new programming language isn't difficult. The difficult part is going to be coordinating your diverse team and getting them all to write decent, coherent code, with comments, and to a given style. I would suggest that this is actually going to be impossible, hence my advice to forget about designing yet another monolithic system and aiming for a framework to support individually written components. That way, individual developers can mess up stuff, leave or join the project, get in huffs and argue with other developers, all without impacting the entire project too badly.
Oh and by the way my team just picked up 2 more programmers, one with mainly knowledge in C and Delphi, and one with extensive Java knowledge.