You're not going to want to learn "one language to support tools for all platforms." You're going to want to learn a great language for each platform you need to support. I know that sounds harder, but it isn't, really – most languages are based at least loosely on C, so their syntax is always similar. But the "one size fits all" approach with languages is like the "one size fits all" approach for clothing. Never fits right.
For Windows, I'd say PowerShell and C#. For Linux, I'd probably go with Python, myself.