I am a beginner in studying Korean grammar, and I noticed the word 한국어를 one day while practicing. 한국 - Korean, 를 - plural, but it doesn't make sense to me. Example: 나는 한국어를 사랑. Why is the word 한국어 plural, and when should I use it as plural and by itself?
As the commenters said, 를 has nothing to do with plurality. 를 is a objective marker particle, meaning it marks the object of a verb, as in 나는 당신을 사랑합니다 = I love you.
You might be mixing up 들 with 를 (note the consonant ㄷ at the top).
들 is the suffix indicating plurality. For example, you can say 사람들 instead of 사람 to mean multiple people. However, 들's role in Korean is not exactly the same as the plural suffix "s" in English. In Korean, indicating plurality is completely optional, which means you only use it when you think it is necessary. Because of this nouns without 들, like 사람, 물건 or whatever, don't mean they are necessarily singular just because they lack 들. The number is either unknown (i.e. could be singular or plural) or known by the surrounding context.
Lastly, you cannot use 들 with 한국어 if that is what you were thinking, because 한국어 means the Korean language which there is only one in the world. 한국어들 just doesn't make any sense.