Ok I just get it. I was using the Dictionary in a totally wrong way.
I used to add the instance variables directly in the dictionary proprieties so I didn't have any key in it. I did't created a Json file fist with all my keys and values.
So here is what I was supposed to do first:
+ System: On start of layout
-> AJAX: Request MyDictionaryFile.json (tag "Dico")
-> System: Wait for previous actions to complete
+ AJAX: On "Dico" completed
-> MyDictionary: Load from JSON string AJAX.LastData
And now this works fine:
-> Text: Set text to MyDictionary.Get(MyArray.At(Object.Line,0))
Maybe it to should be noticed in the manual…