a) where to get them?

b) how to install them?

c) how to add some one?

d) and how to select an other one if possible to change the language within d1) a text? d2) a sheet?

e) is there an indicator (icone or short cut) the the language of the selected dictionary?

f) are the same dictionaries compatible with kate / konqueror (if yes the indicator would be better positioned in the bottom bar and be valid in each application until the next change analog to the analog selections way concerning keyboard, but only one indicator for both would not be enough: some keyboards are compatible with a lot of languages like "us intl": there is no reason to have to write with a it keyboard to permit the text spell checking in it as "us intl" or "ch" keyboard etc can write perfectly the Italian language!)

I'm not sure about Calligra specifically, but kdelibs uses aspell I believe. You can use your package manager to install the appropriate aspell language dictionaries.