For GetMetadata/Dialect the spec says:
--
[Body]/mex:GetMetadata/mex:Dialect
    When this repeating OPTIONAL element is present, the response MUST include only Metadata Sections corresponding to the Dialect specified by the IRI attribute; if the receiver does not have any Metadata Sections of the indicated Dialect, the response MUST include zero Metadata Sections. 
--

Couple of problems with this:
1 - its not 100% accurate. At the Dialect element level to only talk about 
how the response needs to only include metadata of this Dialect URI w/o any
mention of the @Identifier and @Content attributes is misleading.
2 - this paragraph repeats what the definition of the @URI attribute says
3 - the @URI definition text is better since it ends with:
     ...the response MUST include zero Metadata Sections for this IRI. 
    the "for this IRI" removes some possible confusion.  The current wording
    above could be interpreted to mean that if there are multiple
    Dialect elements and any one of them fails to match something then the
    entire response MUST not include ANY Metadata sections at all - even for
    other Dialects.

Proposal:
Modify the above text to be:
--
When this repeating OPTIONAL element is present, the response MUST include
only Metadata Sections corresponding to the Metadata matching the selection
criteria specified. If the receiver does not have any Metadata Sections
matching the selection criteria the response MUST include zero Metadata
Sections for this Dialect element.
--
and leave the definition of each attribute to get more specific.