Many animals have nervous systems, pain responses, learning abilities, preferences, fear behaviour, social bonds and signs of distress. These are strong reasons to take animal suffering seriously.
Animals do not need to speak human language in order to suffer. Human infants, people with severe communication difficulties and non-human animals can all have experiences without explaining them verbally.
There is uncertainty at the edges. But uncertainty does not justify indifference. If there is good reason to believe a being can suffer, then causing unnecessary suffering requires moral justification.
Evidence notes
Evidence includes behaviour, physiology, veterinary science, neuroscience, animal welfare research and evolutionary continuity between humans and other animals.
Ethical questions
If animals can suffer, how should that affect food, farming, entertainment, hunting, research, land use and daily choices?
Conclusion
Animal suffering is real enough to require moral attention. The harder question is whether our habits honestly reflect that fact.