Please follow the CONTRIBUTING guide for general info on how to contribute to Remmina.
Remmina uses xgettext to generate the translated strings, you don't necessarily need to have a good understanding of it but in the case you need an high level overview check this
Using Git and and a text or PO Editor
To add a new language
Download the remmina.pot and using a PO editor (Poedit, Gtranslator, …) create the PO file of your language.
When you are done send it to email@example.com , alternatively you can fork the Remmina repository and submit a merge request.
To update an existing translation.
All the existing po files are under https://gitlab.com/Remmina/Remmina/blob/master/po
Grab the one you want to update and use a PO editor to update it, than you can either send to us at firstname.lastname@example.org, either submit a merge request.