Превод - Португалски Бразилски-Японски - Você também é muito bonita! Eu te amo muito!

Hi Mikhail,

I didn't understand your remark ("I did the translation considering that the writer is a girl between 15 and 18 years old" ).
The line was probably written by a man (bonita = feminine adjective) and nothing gives a clue about his age or the addressee's.

I feel a situation of this phrase where 2 girls are talking.

When I translate foreign language into japanese,i always consider "writer's sex" because i think it is the writer that will use phrase in conversation with japanese people.

Japanese language has many ways of saying a phrase which has same meaning depending on speaker's sex,age,character(cool,not cool,serious,and gentle...).

I understand that. Other languages also require to change some words according to the speaker's gender, however in this case I think the speaker is likely a man talking to a woman.
But what I don't understand is that age you refer to (15-18 ) Why is that?

When I saw the gender mark of requestor,i considered that requestor was women.

Next,a part of the line was written by femine adjective.

At last,i belived that brasileiros says "Eu te amo" to their friends from conversation with a friend who went to study in brazil.

These reasons made me to imagine the situation where 2 girls is enjoying going to shopping in clothes shop.

One girl show her new clothes to her friend,another says that You are cute!! I love you.

From my image,it is girls with 15-18 age that are going shopping!

That's all.

My imagination is always so strange that i feel DISGUSTED after saying the reason why i imaged like that because i have no sense of thinking logically....

OK, let's see...

If we go to the requester's profile, we may actually think the line was written by a woman, however according to the year of birth she stated, she is 31 years old, not a teenager anymore!

Your idea of two girls doing shopping is a bit weird since in that case (one of them talking about the other's looks) the line should be:
"Você também está muito bonita!"

About Brazilians using "Eu te amo muito!", to address a friend... I don't think it is very common. We'd probably say (to a friend, no matter the gender): "Eu gosto muito de você" or more informally: "Adoro você!"
But, of course, it's not impossible that some people, talking to a friend, use "Eu te amo muito!"