I would suggest you to use "SpellCheck" component on RTE. When an application user clicks the SpellCheck button on a user form, the system uses information in the SpellChecker Properties instance (based on the locale of each user) to determine which dictionaries to apply, how to operate the spellchecking algorithm, and other details.
By default PEGA provides 6 dictionaries ( instances of Binary files having *.clx naming ). If you have specific grammar needs for any specific locale, you need to create your own clx file and then specify it in your SpellCheck Property rules. ( DATA-SPELLCHECKER-PROP )
I think Sumit's question is towards checking grammar of a sentence rather than a lexical error. Our existing spellchecker AFAIK checks for spelling errors and additional checks for mistakes in hyphenated and conjugated words by splitting them but there is no component that checks for grammar like MS Word.
(This information is not published on PDN yet. I'll ask the concerned team to make the info available in this space. )
PRPC RTEs or Textareas don't support Grammar check yet. Pega 7 RTEs are based on CKEditor and although CKEditor has a few plugins for grammar, i don't think they work seamlessly. Its not possible to integrate 3rd party plugins with PRPC and this is something we will need to ship with the OOTB RTE. You could submit an enhancement request for the Product/Engineering team to look into.