Thank you all!
apt-get install collaboraoffice5.3-dict-en
apt-get install collaboraofficebasis5.3-en
apt-get install hunspell
apt-get -y install locales-all
systemctl restart loolwsd.service
The spelling check function is working now.
But here comes another issue:
For example: two misspelling words
Try to correct the first one, the “firrst” is in red color and suggestion show “first”.
then, after clicked the “Correct” button
"seconnd" does not turn red, but the suggestion field shows the correct one.
It seems that “Not in dictionary” field is not updating.