Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency care for a sudden inability to speak or understand language, particularly with:
Seek prompt medical advice if your child gradually loses words or communication skills they previously used.
Regression, meaning a loss of established skills, needs assessment even if your child otherwise appears well.
These sudden symptoms are different from a developmental language disorder and may signal an urgent neurological problem.
Arrange a routine assessment if you are concerned that your child is not understanding or using language as expected, frequently becomes frustrated when communicating, or is struggling with conversation, reading, writing, or school instructions.
You do not need to wait until a child is failing at school.
The article also says
A hearing check is appropriate when language development is delayed, especially if your child does not respond consistently to sounds or has repeated ear problems.
Every sign above is quoted from the sofidex article on Language disorders in children. If you are not sure, err upward — it is always reasonable to be checked.