Are expenses of monthly language-immersion parties I host with my language students deductible? I'm self-employed. If deductible, under what category should I list them?

If your business is teaching languages, then yes the expenses are deductible.  You can list the total cost of the event as an expense of its own as Other Miscellaneous Expenses or you can divide out the costs you incur and book them according to the category they each represent (meals as meals, facility rent as rent, etc).

Also, be sure you are deducting all the expenses you are eligible for by reviewing What self-employed expenses can I deduct?