Creative Commons Attribution 4.0 International (CC BY 4.0)
Reasoning:
Nature of the Project: The inclusion of software and tools in addition to the specification makes the licensing a bit more nuanced. However, since you mentioned an emphasis on the data standard and interoperability, a Creative Commons license might be more appropriate for the data standard, while using a dual licensing approach for the software/tools.
Contributions: The CC BY 4.0 allows and encourages contributions. Its nature and intent make it clear for derivatives and adaptations.
Usage & Distribution: The CC BY 4.0 license does not place restrictions on commercial use or derivative works, but derivatives do not have to be open-sourced, aligning with your preference.
Commercial Use: This license permits commercial use without imposing obligations to give back, though users can be encouraged to do so.
Credit & Attribution: This is a fundamental part of the CC BY 4.0 license. Anyone using the work must give appropriate attribution.
Compatibility: While the CC BY 4.0 license does not impose open source conditions on derivatives, it ensures broad interoperability and is friendly to both open and proprietary use-cases.
Protection & Liability: The license includes a disclaimer of warranties.
Simplicity & Understanding: CC licenses are widely recognized and understood in both commercial and non-commercial communities internationally.
International Use: The "International" in CC BY 4.0 ensures that the license is crafted for use around the world, regardless of local copyright laws.
Updates & Versioning: The license does not restrict usage to specific versions of your standard.
MIT License It's permissive, allows for commercial use, doesn't require open sourcing of derivatives, but does require attribution