In Chinese Emperor is translated into 皇帝, whereas king into 國王. 皇帝 belongs only to the Emperor of China, while 國王 was inferior to 皇帝. Was that the same in Europe?
Or, for instance, was the title "Emperor/Empress of India" superior (more important and prestigious) to "King/Queen of Britain"? If so, why didn't the government also elevate the leader of Britain to "Emperor/Empress of Britain"?
Traditionally the imperial title of emperor is regarded as superior to that of king.
This is discussed by u/aeoleth in this thread https://www.reddit.com/r/AskHistorians/comments/ke6w0g/according_to_wikipedia_apparently_the_title_of/
Translating titles from different languages is often challenging and very context dependent. This is discussed by several people in this thread:
https://www.reddit.com/r/AskHistorians/comments/6axu3w/how_do_historians_decide_what_modern_english/
It's important to remember that context can be key to the translation. The same term in the original language can be translated differently. An example would be the Greek word romanised as Basileus, which can be translated as king or emperor depending on time period and location.
On the specific point of Victoria being declared empress of India while remaining queen of the United Kingdom of Great Britain and Ireland, this thread has an excellent answer from u/noodlebloom