> When I change a single word in that file, the Spanish translation does not load anymore 

The translation file is a JSON file so make sure you do not make a syntax error (e.g. missing double quotes, commas). Also make sure the file is saved with UTF-8 encoding (e.g. if you open it with Visual Studio it should keep the encoding but Notepad may not)

Are you sure you adding the new properties in spanish translation file in correct places like this:

{
  "controlbar": {
    ...
    "find": "Find",
    ...
    },
...
  "sidepanel": {
    ...
    "searchTab": {
      ...
      "anyWord": "Any word",
      "wildCardMode": "Wildcard search",
       ...
    }
  },
  ...
 }

Also note that "translation-es.json" file name should work if the file is correctly formatted (not needed to be "translation-es-AR.json" if it's not specific for es-AR)

Thank you for your reply. I'm familiar with the JSON syntax. I also validated the JSON file with JSONLint, just in case, found a couple of errors in the original translation-es.json file (repeated items in a couple of collections). Fixing those errors won't help. ANY change I make to the translation-es.json file results in the Spanish language not loading.

 I also checked the file encoding. Notepad++ reports it's UTF-8 with boom.

Strange, did you use the latest translation files as specified in here ?

Also did you  try to explicitly set Language property:

documentViewer.Language = "es";

Normally it's detected automatically from Thread.CurrentThread.CurrentUICulture but you can force the language ("es" or "es-AR") by setting this property.

The currentculture is properly set and the DocumentUltimateControl is shown in Spanish, unless I "touch" the "translation-es.json" file in the ".\App_GlobalResources\WebViewer\i18n" folder.