Conditionals have three forms.
If you work hard, you will succeed.
If you worked hard, you would succeed.
If you had worked hard, you would have succeeded.
If you think about their meanings, you will understand why the tenses correspond to each other like this.
In #1, the cause is in present, in control, the result in future, unknown.
In #2, the cause is in present, though past tense is used, but it's not in control. Kinda like a teacher reprimanding a student after bad grade in one small quic.
In #3, the cause was in past, absolutely out of control. And the result is also in past, out of control obviously. Kinda like when a teacher shakes her head after a student has failed the course/class.
Let's look at your sentence now,
I would understand everything you said if you said it in Portuguese.
I think it should be:
I will understand everything you say, if you say it in Portuguese.
I would understand everything you said, if you said it in Portuguese.
The sentence you mention is correct as it is, but depending on what the situation is (Think teacher/student: beginning of semester, just a quiz, or end of semester), you can fit in one of the conditionals.