Property files typically holds localized texts.
Therefore their spellchecking makes big sense.
There is one common pattern used for them:
everything in '{}' brackets can mean MessageFormat
formating templates.

How can I call spellchecking API? I'm looking for
something like:

  Iterator wordsIterator = <create iterator of
                           Words to check>
  SpellChecker spell = <get spell checked for
                       a locale>
  // show UI and iterates over replacing mispelled
  spell.spell(wordsIterator);  // SpellChecker API

where interface Word can look like:

// SpellChecker API
public interface Word {
  // show word in editor (to get a context to user)
  showWord();

  // replace by corrected value
  replaceBy(String replacement)
}

Could you introduce org.netbeans.api.spellchecker
API, please?