Slurred speech, mix-up of words. Is that a possible concussion after effect?

Yes. Yes. These would represent rather severe symptoms of a concussion. A stroke or other brain conditions could cause these symptoms. Don't delay in seeking medical treatment for these symptoms. After a neurological exam, then a ct or MRI of the brain may be necessary.
Could be. Sounds like you or whoever had these symptoms should be seen by a doctor who has training in concussion management.