UInt64 and cls compliance

I'm creating a binary format for a custom serializer I'm writing. (I need to maintain the format even on different versions of .Net where binary formatting might change.)

Several of the fields in the file indicate the length of various parts of the data which may get fairly large. I'd like to use an unsigned 64-bit integer (UInt64) to store these values (they will be stored as 8 bytes in the file, not as a string of the number).

The MSDN document on UInt64 and ULong says that neither are CLS compliant. Should I take this to mean that other .Net languages will be/are unable to use UInt64? There are no plans to port this app to a mobile device or any non-windows system.

It's not necessarily that they won't. It's just that they don't have to to call themselves a .NET language.