I just installed the plugin language nl-NL.

Works OK, but I found some grammar errors which I corrected in the file messages.po
I am unfamiliar with this extension, but after saving the corrected file I could not upload in Filezilla.

Status: Invalid character sequence received, disabling UTF-8. Select UTF-8 option in site manager to force UTF-8.
Response: 550 messages.po: Permission non accordée
Error: Critical file transfer error

When I "force" UTF-8 I get same error.

Any advice on how to get it right?

Actually I solved by making the changes on the messages.po file directly on the server.

and OOPS, I put this in the wrong forum. Sorry.