Pretty sure that this would work, or at least get you started.
=iif(Fields!Language.Value="Spanish"
,Switch(isnothing(Fields!SpanishDate.value)=true,"Message 1"
,isnothing(Fields!SpanishTime.value)=true,"Message 2"
,isnothing(Fields!SpanishLocation.value)=true,"Message 3"
,1=1, "Good Spanish Message")
,Switch(isnothing(Fields!Date.value)=true,"Message 4"
,isnothing(Fields!Time.value)=true,"Message 5"
,isnothing(Fields!Location.value)=true,"Message 6"
,1=1, "Good Message")
)