Lista ingredientelor arată glutenul.

They are:

• lista → listă = list
• ingredientelor → ingredient = ingredient
• arată → a arăta = to show / to indicate
• glutenul → gluten = gluten

So the sentence is built from very normal dictionary forms, just with the endings changed to fit the grammar.

Because lista is the definite singular form of listă.

• listă = a list
• lista = the list

Romanian usually adds the definite article to the end of the noun, instead of putting a separate word like the in front.

So:

• listă → indefinite
• lista → definite

Romanian often expresses of by using the genitive case instead of a separate word.

So:

• lista ingredientelor = the list of the ingredients

Here, ingredientelor is in the genitive plural form, which gives the meaning of the ingredients.

This is very common in Romanian:

• ușa casei = the door of the house
• culoarea mașinii = the color of the car
• lista ingredientelor = the list of the ingredients

Because ingredientelor is the genitive plural definite form of ingredient.

The rough pattern is:

• ingredient = ingredient
• ingrediente = ingredients
• ingredientele = the ingredients
• ingredientelor = of the ingredients / to the ingredients

In this sentence, it means of the ingredients because it depends on lista.

So lista ingredientelor literally means the list of the ingredients.

Yes. Lista de ingrediente is also correct, and in many real-life contexts it is actually very common.

There is a small difference in feel:

• lista de ingrediente = ingredient list / list of ingredients as a general label or category
• lista ingredientelor = the list of the ingredients, sounding a bit more specific

So on packaging, lista de ingrediente is often what you would see as a heading, while lista ingredientelor is also grammatical and understandable.

Arată is the 3rd person singular present of a arăta.

That means it matches a singular subject such as lista.

So:

• eu arăt = I show
• tu arăți = you show
• el/ea arată = he/she/it shows

Here the subject is lista ingredientelor, which is singular because the head noun is lista. So arată means shows / indicates.

Glutenul is the definite form:

• gluten = gluten
• glutenul = the gluten

In Romanian, the definite form is often used where English might still simply say gluten. So even though Romanian has glutenul, the most natural English translation may still just be gluten, not necessarily the gluten.

In other words, the Romanian grammar marks it as definite, but English may translate it more naturally without the.

Because pe is generally used with direct objects that are people or sometimes highly specific animate beings.

Examples:

• Văd copilul.
• Îl văd pe copil.

But glutenul is an inanimate thing, so Romanian normally does not use pe here.

So:

• arată glutenul = correct
• arată pe glutenul = not correct

The main clue is the usual Romanian word order: subject + verb + object.

So:

• Lista ingredientelor = subject
• arată = verb
• glutenul = object

Also, this is the most logical meaning: the ingredient list is the thing that shows something, and glutenul is what is being shown.

Romanian word order can be flexible, but in a plain sentence like this, the first noun phrase before the verb is normally understood as the subject.

It is understandable, but it may sound a little less natural than some other ways of saying the same idea, especially in a food-label context.

More natural alternatives might be:

• Lista ingredientelor indică prezența glutenului.
• Glutenul este menționat în lista ingredientelor.
• Pe lista ingredientelor este indicat glutenul.

So your sentence works as a grammar example, but in real packaging or formal wording, Romanian often prefers a verb like indică or a phrase like prezența glutenului.