When I try to use DictionaryWordQ["word"] I get a list of errors:

Get: Cannot open SpellingData`.
Needs: Context SpellingData` was not created when Needs was evaluated.
Keys: The argument SpellingData`SpellingDataDicts[] is not a valid Association or a list of rules.
DictionaryWordQ: Spelling dictionaries for English are not available.

I assume that no dictionary comes with Mathematica distribution on Raspbian
How can install a dictionary? Or there is some other problem?

Yes the spell checking dictionary is missing and spell-checking is not in the Edit menu as it should be.

The spelling dictionnary should normally be in
/opt/Wolfram/WolframEngine/12.0/SystemFiles/SpellingDictionaries
but it is not.

Many things are broken in the Mathematica front end for Raspberry Pi (v12).