In other words you both are using the same type of equipment. Interoperability is when you are using a radio that is US made and he is using a radio that is French made, and they both conform to the same set of parameters, such as frequency, mode, power, etc. The language barrier can be overcome using other means such a s code word systems. These are type of problems are addressed through Standardized Operating Procedures, etc. Since we're adding our two-cents, here's mine.
Interoperability is critical, as is reliability, etc etc.
IMHO, DISA needs to look at ways to speed up its testing and certification processes. Rapid prototyping, Spiral development, think outside the box etc etc. Seems as though this and several other postings did not understand the definition and clarification provided by Major Nierle. This means that it includes the technical, procedural, doctrinal and operational interface and the use of the information exchanged. In the below example, interopability does not exist unless both parties understand the information exchanged and can use that information to operate effectively together.