Foreign Language in WordPress

Hi, I’m working on a wordpress site for someone, and they would like all of the text translated into a language other than English. Almost all of the text I’m editing is in PHP files, and I’m assuming most files for wordpress are php files. How do I write the special characters into the site? When I try just putting the characters like Ç or Õ or any other character with a ~ or anything else, all the text disappears. Thanks,