In Spanish (es_ES) and Swedish (sv_SE), the shortcut Shift + R is assigned to both Open In Reference View and Constrain Aspect Ratio. Typing Shift + R always invokes Open In Reference View even when the Crop Tool is open.
The relevant TranslatedStrings lines:
"$$$/AgDevelop/Menu/View/ReferenceActiveView/Key=Shift+R"
"$$$/AgDevelop/Menu/Photo/ConstrainAspectRatio/Key=Shift+r"
All languages assign Shift + R to Open In Reference View.