Support the use of cell references in language specification

[csnyulas]

It would be really helpful if one would be able to specify the value of the xml:lang directive as coming from the spreadsheet. For example, if we want to create a multilingual ontology, and we have a spreadsheet containing in different columns the labels in different languages for the same class (each row representing a class, while the header row contains the value of the language tags, like "en", "en-AU", "pt", "it", "it-CH", etc.), it would be cool if we could write a rule like

Class: @A*
  Annotations: rdfs:label @**(xml:lang=@*1)

Currently, AFAICT, this is not supported, and the values of the xml:lang directive need to be specified explicitly, as a quoted string literal. So, the above rule, needs to be written something like this

Class: @A*
  Annotations: rdfs:label @A*(xml:lang="en"),
    rdfs:label @B*(xml:lang="en-AU"),
    rdfs:label @C*(xml:lang="pt"),
    rdfs:label @D*(xml:lang="it"),
    ...

, and whenever a new column is added the rule needs to be modified.