Hey, America. If you are going to ignorantly continue to use your obsolete and impractical system of measurement in spite of the rest of the would moving on to an objectively superior system generations ago, could you at least spell litres correctly when you fucking use the word?
Well, here's the thing with language, it is whatever people who use the language use. If you can spell litre as liter and it's widely accepted, welp, liter is a correct and valid form then.
Litre is an international scientific standard. It's spelling is not up for debate. Why don't you just change It's volume as well, and completely fuck up all scientific communication while your at it.
The spelling of the word, much like any and all words, changes based on how it is used by the people. Standards and definitions follow the usage. It's not about debate, that's literally just language. You can already see this reflected in many sources, such as Wikipedia here:
The English spelling and even names for certain SI units, prefixes and non-SI units depend on the variety of English used. US English uses the spelling deka-, meter, and liter, and International English uses deca-, metre, and litre. The name of the unit whose symbol is t and which is defined by 1 t = 103 kg is 'metric ton' in US English and 'tonne' in International English.[4]: iii
If we're talking about the order the sounds are made, "liter" is more correct. I never understood why Europeans spell the "er" sound as "re". It's just now how the sound works.
My take is that spelling should reflect the sound. In any language. For every word, every time.
American English makes a ton of errors in this regard, you'll get no argument from me there (for example any word with "ough" or "augh" is automatically spelled wrong).
I'm sure tons of other examples in pretty much every language make the same mistake. But as far as I can tell, there is no good reason the spelling shouldn't be a representation of the exact order of sounds that make up the word.
All that to say, even when hearing people who speak all manner of different languages use the word "liter", not one has ever pronounced it "litre".
Honestly it should be more like "ledur" for most Americans. We don't have a habit of the actually making the proper "t" sound very often. But I'm getting into a whole different argument, so I'll leave that kinda rant for a different time.