Alleen = Only ?

That is a very common mistake in translation. I used it myself for many years. Dutch people are generally very tolerant, and they do try to understand you without making it a big deal.

However, many small words—such as “alleen”—can completely change the tone, meaning, and implication of a sentence when they are used incorrectly. That is why it is worth being a little more careful with them.

Alleen can mean only. But only with the sense of restriction, or exclusivity.
Ik drink alleen koffie.
This means: I drink only coffee—not tea, not juice, not sparkling water, and not any other option on the menu. It describes a general limitation or preference.
It does not mean: I only want to drink a cup of coffee now. (That is : Ik wil graag alleen koffie.)
Ik eet alleen maar boord.
This means: I only eat bread. — This is stronger than only using “alleen”. I can hear the complaint, emphasis on the “only” in this sentense and sounds quite negative.
Ik ben gewoon moe.
This means : I am just tired. — just / simply / basically (pragmatic word). This word can affect the tone of the sentense. In English you might say, I am just tired… But you never say : Ik ben alleen moe.
Hij is pas 18 jaar.
This means: He is only 18 years old. — time-based tone, and not as expected. Here means he might look like 20, but turns out he is only 18.
Het kost maar vijf euro.
This means: It costs only 5 euro, no more. — “maar “mainly minimizes or downplays, often emotionally or context-dependent.

The common mistakes are :
Ik wil alleen één. ( It should be : Ik wil er graag maar één.)
Ik wil een appel alleen. (It should be : Ik wil alleen een apple.)
Ik wil alleen maar één. ( It could be : Ik wil er alleen maar één. But be careful with the tone that you want to express.)