Diploma in Korean language is a 1 year course which you will be doing which will be only counted as 1 level, there are chances to get job , better have a certified course. Many websites/Apps offer you courses on Korean language such as

1. Eggbun.
2. Pop Popping Korean.
3. Coursera.
4. Naver Dictionary
5. Learn Korean Phrasebook

many other also provide the same , please choose according to your needs , other option is that you can learn through youtube for free and take a Korean language exam in order to get certified

hope it clarifies you ,

better wishes for your future