"Your baggage is there!"
Translation:Οι αποσκευές σας είναι εκεί!
No, not always, but unless you know you are talking about one item only (e.g. one extra item of baggage - επιπλέον αποσκευή, (for one item) baggage size - το μέγεθος της αποσκευής), the plural is used. When talking about a specific item you also usually specify what type of luggage it is, e.g. βαλίτσα.
Yes, "η αποσευή" is the singular. However, the English "baggage" which is to be translated here is uncountable but refers to a plural number.
Otherwise, we would say "piece of baggage".
"Hair" is also uncountable but we usually have more than one and we still say..."My hair is..."
"Your furniture is beautiful." another plural item that is uncountable and uses "is" although we are referring to more than one item.
"All his advice was useful."
"This is the information I received, it's all important."
Here is a list of uncountable items, many of which represent multiple pieces, that all use a singular verb.