Vertimas - Portugalų-Senoji graikų kalba - Ainda que as águas rujam e se pertubem,ainda que...

The translation is perfect, being actually the text of the psalm from the 'Septuaginta', as I think is more proper in such cases. The last word (ἐστὶν) isn't needed, it's not in the 'Septuaginta' either. This is not a mistake, but I suppose when we "translate" bible verses in Greek it is better to keep the original septuaginta wording.

Yes, of course, we can remove ἐστὶν from the translation.