fix(service): check whether browserLang is undefined

[cfilipponi]

When accessing a non-existing property, JavaScript returns undefined.
The check for 'undefined' will guarantee that browserLang has been and we can safely call .indexOf()

[cfilipponi]

Another approach would be wrapping everything in a conditional like this:

if (browserLang) {
      if (browserLang.indexOf('-') !== -1) {
        browserLang = browserLang.split('-')[0];
      }

      if (browserLang.indexOf('_') !== -1) {
        browserLang = browserLang.split('_')[0];
      }
    }