Why don't you grow up?
Retina, 150 bytes
[ÀÁÂàáâ]
a
[ÉÊéê]
e
Í|í
i
[ÓÔóô]
o
[ÚÜúü]
u
T`L`l
ão|õe
im
[ck][aou]|qu[ei]|ke
ki
ç[aeiou]|ce
ci
g[aou]|gue
gui
j[aeou]|g[ei]
ji
(gu)i|[aeou]
$1i
ii
i

Try it online!
This is my first Retina program, so there's probably a lot of improvements to make. Suggestions are welcome.
Thanks to Martin for correcting some mistakes and for some golfing suggestions.
Explanation
I'm going to break up the explanation into 3 sections, to make it easier on me.
Section 1: Preprocessing
[ÀÁÂàáâ]
a
[ÉÊéê]
e
Í|í
i
[ÓÔóô]
o
[ÚÜúü]
u
T`L`l

The first 5 rules replace any accented vowel with the corresponding unaccented version. It's shorter to list out the matches in character classes (or, in the third case, as two options) than to use the culture-invariance and case-insentivity options each time. The last rule converts all ASCII letters to lowercase.
Section 2: Mimimification
ão|õe
im
[ck][aou]|qu[ei]|ke
ki
ç[aeiou]|ce
ci
g[aou]|gue
gui
j[aeou]|g[ei]
ji

These rules simply apply the substitutions specified.
Section 3: Cleanup
(gu)i|[aeou]
$1i
i+
i

The first rule matches any vowel except for the u in gui, and replaces them with is. The second rule removes repeated is.