I’m a special education preschool teacher and I work with lots of kiddos who have speech delays. An unfamiliar listener (someone who doesn’t spend a lot of time around d the child) should be able to understand about 50% of what they are saying. They should also be be speaking in 2-3 world phrases to express their wants and needs. If you have concerns and are in the US, you can reach out to your local school district and request and evaluation for special education services. They will evaluate your child and if they qualify, will receive therapy at no cost through the district.
I know a 4 year old that doesn’t speak much 🤷🏼♀️