Sarql query - How to deal with available languages

[ghost]

The available languages for a property vary from object to object. For instance, the following query will work if we ask for "FR" but won't work if we ask for "EN" (the item labels are available in french and english, but the classification label exists only in english):

 PREFIX skos:<http://www.w3.org/2004/02/skos/core#>
 PREFIX xkos:<http://rdf-vocabulary.ddialliance.org/xkos#>
 SELECT ?label ?clLabel ?parentLabel {
    <http://stamina-project.org/codes/cpfr21/categorie/47.00.1> skos:prefLabel ?label ;
                                                                skos:inScheme ?cl .
    ?cl skos:prefLabel ?clLabel .

    <http://stamina-project.org/codes/cpfr21/categorie/47.00.1> skos:broader ?parent .
      ?parent skos:prefLabel ?parentLabel .

    FILTER (langMatches(lang(?label), "FR") && langMatches(lang(?parentLabel), "FR") && langMatches(lang(?clLabel), "FR"))
  }

Note: this is a simplified version of itemDetails query.

[ghost]

The issue is more global. The problem exposed above occurs when the same language is not present for all the properties. But more generally, if we suppose we have a default language, do we provide a fallback when the properties are not available in this language ? For instance, the query above won't work in french but will work in english:

 PREFIX skos:<http://www.w3.org/2004/02/skos/core#>
  PREFIX xkos:<http://rdf-vocabulary.ddialliance.org/xkos#>
  SELECT ?label ?code ?label ?text ?cl ?clCode ?clLabel ?parent ?parentCode
      ?parentLabel {
    <http://stamina-project.org/codes/nacer2/section/I> skos:prefLabel ?label ;
              skos:notation ?code ;
              skos:inScheme ?cl .
    ?cl skos:prefLabel ?clLabel .

    OPTIONAL { 
      ?cl skos:notation ?clCode .
    }


    OPTIONAL {
      <http://stamina-project.org/codes/nacer2/section/I> skos:broader ?parent ;
                skos:prefLabel ?parentLabel .
      OPTIONAL { ?parent skos:notation ?parentCode . } 
    }      

    OPTIONAL {
     <http://stamina-project.org/codes/nacer2/section/I> xkos:coreContentNote ?content .
     ?content xkos:plainText ?text .
    }
    FILTER (
      langMatches(lang(?label), "EN") &&
      langMatches(lang(?clLabel), "EN"))
  }

One option could be to deal with the language in the ui, where we could implement some specific rules to have a safe fallback. But it's not trivial. If we do so, we need to remove the FILTER statement in queries and build a process utility. Another option could be to make dedicated queries to retrieve all text properties: these queries should return one or multiple results depending on the languages available. But it adds a lot of complexity too.