This is a GREAT question! I'm sure we've gone over a few versions of these but I thought it would be helpful for you guys to have a couple of phrases you can use in situations like this! (even though all of your Korean is good cuz of TEUIDA~ hehe)
1. ํ๊ตญ๋ง ์ ๋ชปํด์. (han-guk-mal jal mot-hae-yo)
2. ํ๊ตญ๋ง ๋ชปํด์. (han-guk-mal mot-hae-yo)
The difference between these two is the ์ which means (good). The first sentence means "My Korean isn't very good" and the second one means "I can't speak Korean"
Both could be used but if you are just warning them I would use #1 and if you want to end the conversation by saying you don't speak English OR asking for another language then use #2!
3. ์ฒ์ฒํ ๋ง์ํด์ฃผ์ค ์ ์์ด์? (cheon-cheon-hi mal-sseum-hae ju-shil su it-na-yo?)
OKAY this one is for you advanced ppl. I know it is a long phrase but it is the longer version because this is the MOST polite way to say this. This phrase means "Could you speak slower please?" and some websites and textbooks will teach you: ์ฒ์ฒํ ๋ง์ํด ์ฃผ์ธ์ but it just means "Speak slowly please." Which is great! BUT I figured I would teach you guys the more polite version~
HOPE THAT HELPED ๐