It’s easier to see the \ characters in the GitHub repository: bilara-data/translation/es/maggatr/sutta/sn/sn3/sn3.3_translation-es-maggatr.json at unpublished · suttacentral/bilara-data · GitHub. In Bilara they don’t show, but they go away when you delete the invisible “character” that comes before the character following the \ in GitHub. They sometimes come in when you copy/paste things from elsewhere.
For quote marks and diacriticals and such things, it depends on your operating system. I am using a Linux system, here I can define a “compose” key that is then used to build such special characters. For other systems there has been some discussion here already a while ago, and I don’t know what are the most up-to-date solutions. Maybe Carmi has some advise on that too.
There’s also some information on typographically correct quotation marks here, including a table with the correct typography in various languages, including Spanish.