Copy and modify the gopher64.pot file in this directory with your translations.
Rename it to gopher64.po and put it here: data/translations/<lang>/LC_MESSAGES/gopher64.po

See here for more details on creating a .po file. I recommend using a program like poedit to create the .po file from the template.

For example, in the .pot file you will see:

#: src/ui/gui/about_page.slint:17
msgctxt "NewVersionButton"
msgid "New version available! Click here to download"
msgstr ""

A translation will look something like this:

#: src/ui/gui/about_page.slint:17
msgctxt "NewVersionButton"
msgid "New version available! Click here to download"
msgstr "Nova versão disponível! Clique aqui para baixar"

The only line you modify is the msgstr. The msgctxt and msgid need to remain unchanged.