We’ll have lots of English speakers here given the language the question was asked in, so I’ll do Gàidhlig (Scottish Gaelic) instead: dìochuimhneachadh, at 17 letters. It means “forgetting”, and it is pronounced /ˈd̥ʲĩə̃xənəxəɣ/. No, I can’t say it smoothly.

Gàidhlig isn’t one of those languages that can compound words like Finnish or German, this one is just a consequence of a few different things. Firstly, the language’s spelling rules result in a lot of letters that do impart information but aren’t directly pronounced. Consonants have two forms depending on which of two sets of vowels they are next to, so any consonant or consonant cluster must always have vowels from the same set on either side. For example, the “i” in the “imhne” bit in the middle is basically only there to match the “e” at the end, since u and e aren’t in the same set of vowels and we need to know which version of the consonants between them to use. Every h is a modifier on the consonant preceding it as well. Second, the root of it is “un-remember”, so it’s already a shorter word with a prefix. Third, we’re using the verbal noun version, so it’s “the act of forgetting” rather than present-tense as in “currently forgetting something”

There are probably longer words in the language, but I don’t know it very well yet and this was the longest one I could find on a word list. I think there’s actually a version of dìochuimhnich that includes a suffix marking it as being a conditional first person plural doing the forgetting, so “we would forget”, but I don’t understand how that part of the language works. If I was to say that at the moment, I would use two words to do it, so I don’t feel like I can give it as an answer here