For a new translation, use scummvm.pot (from [https://github.com/scummvm/scummvm/tree/master/po the source code repository]) as a template and fill out the header. Then you just have to translate the strings and save your work in a po file. To download the file, click on scummvm.pot and then the ''raw'' or ''View Raw'' button. If this tries to display the file in your browser, do a right click on that button and select ''Save target file as...'' (the working might be different depending on the browser). You might also need to remove any extension added by your browser (for example for me Safari downloads the file as ''scummvm.pot.pps'', apparently believing it to be a MS Powerpoint file).
If a translation already exists for you language, you can get the up to date po file from [https://github.com/scummvm/scummvm/tree/master/po this same source code repository]. As for the template file, you need to click on the file you want and then use the ''raw'' or ''View Raw'' button. You may need to do a right click on the button to save the file on your computer instead of displaying it in the browser. Also some browser may add a ''.txt'' extension that you will need to remove. Before proposing changes to the team (or committing changes if you have commit access) you should let the primary translator know of the changes. You will find contact details for the primary translator(s) in the header of each po file.