I found out that Gantt diagrams in PlantUML offers the possibility to display the project on a calendar instead of an array of numbered columns. This is a great idea, but it lacks of international support: months and days abbreviations will be generated in English.
Users should be able to select a language or to redefine the existing words. The syntax in the first case could use the ISO code of the language - my suggestion for the syntax tries to respect the natural language used for these diagrams. For example, for French from France :
Language is fr_FR
In the second case, there could be something like :
"september" changes to "septembre"
"Fr" changes to "Ve"