Translation:I eat sweet bread.
It does come directly from Portuguese (along with many other loans words).
The Portuguese actually have a unique place in Japanese history, being the first European nation to make contact with the country back in feudal times. Source: https://en.m.wikipedia.org/wiki/Japan–Portugal_relations
For い adjectives, the plain form of which ends in -い, and the structure is Adj. + N. : 甘いパン
For な adjectives, the plain form of which doesn't necessarily end in -い, the structure is Adj. - な + N. : きれいな景色（けしき）(beautiful scenery)
The -い or -な in adjectives are not particles, they are just suffixes, you can see them as declension.
Note that きれい (beautiful) and きらい (hateful) are な adjectives though end in -い.
Try using を right after a direct object, which is the thing or person that is being acted on, the "victim" of the action so to speak. When the verb is "to eat", it is safe to say that the food is the object. If you use が (subject particle) instead, that means the bread is eating. That would be a scary sight!
No, "I am eating" is present progressive tense, meaning that the action of "eat" is happening right now. The Japanese sentence is simple present/non-past tense, meaning that the action of "eat" is either a habitual action (making no comment of when it happens, only that it does) or a future action.
Present progressive tense in Japanese would be 食べています.
Your answer is technically correct; you should suggest it as an acceptable answer (using the report function, not the comments).
As for the use of "-tachi", it signifies a definite plural (e.g. "the sweet breads"), not indefinite plurals (e.g. "sweet breads"). It doesn't matter if the noun you're plural-izing is foreign or not, but in general, it is reserved for animate objects only, specifically people or animals. Using it with inanimate objects invokes an anthropomorphic image.
Yes, "I'm eating" is the present progressive tense*, which describes an action that is currently happening. The equivalent Japanese verb tense is 食べています.
*With the right context (example below), it can be interpreted as simple future tense, in which case it's an appropriate translation, but generally speaking, it's best to try to avoid inferring too much about the context with these exercises.
- A: Ugh, it's only 10:30. I can't wait for lunch. What do you have for lunch?
- B: I'm eating sweet bread today.