How to design a database with rare multiple values
If it was me who doing the design what I will do is create a table with name nationality each nationality have ID  ...and since the the user have multiple nationality ...in the user table I will create two columns (foreign key) for nationality nationality1 and nationality2 referencing the nationality table .

the above scenario in case for two nationality .

for multiple nationality (unlimited) check the below scenario 

the scenario for this situation will be similar to Customer and orders (one-to-many) ...one customers have many orders ...you will need 3 tables user nationality user_nationality in the user_nationality table you will the following columns id,user_id,nationality_id ...the user_id is foreign key column referencing the user_id in the user table and the nationality_id is a foreign key referencing nationality_id for the nationality table .

hope that will help .