"a cup of tea"
Yes, but you cannot say "a tea" in English, so that's the point they try to make everyone notice here. If you say "un ceai", you can understand it as a "cup of tea". Just like Spanish, you can say "un té" and that means "una taza de té".
Actually you can say in English "A tea". You might have an exchange like: "Do you want a drink?" "Yes please!" "A tea or a coffee?" "Tea please."
Or someone might say: "Oooh, I could murder a tea right now!" or "I'm gasping for a tea!" The use of the words 'murder' and 'gasping' are figurative colloquialisms here, basically the person would really like a cup of tea.
It is the context that differentiates the usage from something like: "Lapsang Souchong is a tea originally from China." meaning it's a type of tea. In Romanian how would you say that?
Here I'm referring to English as spoken in the UK, the US or other countries may be different.