… in order to display dialog controls according to RTL reading context
I’m afraid, this information is not exposed 
- you likely need to create a list of locales with their RTL statuses, and compare the UI language (from org.openoffice.Setup/L10N/ooLocale, with fallback to org.openoffice.System/L10N/UILocale (?))… and come with an enhancement request 
Possibly @erAck has a better idea?
Nah, AFAIK there’s no such thing exposed to the API.
However, aren’t dialog controls already automatically displayed RTL if the UI language is RTL? Or does that not happen for extension dialogs? Which I’d consider either a bug or a feature to be implemented…
My intent is to shift the layout of controls in a dynamic dialog, it’s not related to controls RTL behaviour yet …
I can only give you a list of currently supported RTL languages:
ar-* Arabic
he-* Hebrew
yi-* Yiddish
ur-* Urdu
fa-* Farsi
ks-Arab-* Kashmiri Arabic script
sd-* (sd-Deva-*, sd-Arab-*) Sindhi
ug-* Uighur
ky-* Kyrgyz
nqo-* Nko
ckb-*, sdh-*, ku-Arab-* Kurdish Arabic script
hu-Hung-* Hungarian Rovas
ms-Arab-* Malay Arabic script
Hope that helps.
Thanks for that list