Under 92.052 of the TX Property Code, L must make a diligent effort to repair a condition if: tenant complains to the person who collects the rent, tenant is not delinquent in rent, AND the condition materially affects the health of safety of an ordinary tenant. Unless you can get a building inspector to certify that excessive cat smell is a threat to your health, you can't use 92.052 to make L fix th problem.
You'll have to fall back on the terms of the lease. Every lease includes a "quiet emjoyment" clause -- L must neither act nor fail to act in a fashion that disturbs T's right of quiet enjoyment. In your case, L's failure to make your neighbor clean the cat box is disturbing your quiet enjoyment of your apt. You'll have to sue L for breach of lease & request damages (the difference between the rent you pay & what you should pay given the smell). You can also request that the court hold that the lease is terminated by L's breach. If you win, you get to move out & L has to refund your deposit.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.
