How do I know which resource bundle (more over, which language) I'm using? (Java)

Question

So, I want to write some tests in selenium. At one point I get a message from a dialog and I want to know if the used text in the dialog is the right one. So I want to check it out in my resource bundle and set the strings both equal. The problem I got is, that the text from the dialog is english and the one from my bundle is german, because that's the default language from my bundle. I know I can just change the local, but someone else might has a different language setting, when they want to run my tests, so they need to adjust the code. But that is not the solution I want.

So my question is: How do I know which language my (I guess) browser is using, so I can get the right value from my bundle?

Heres the I code I use to get the value from my resource bundle.

/**
     * Returns a value from the resource bundle. If the bundle doesn't contain the key, the method will fail.
     *
     * @param key The key.
     * @return The value to the key.
     */
protected String getValueFromResourceBundle(final String key) {
        final ResourceBundle bundle = ResourceBundle.getBundle("base_example_name", Locale.getDefault());

        if (!bundle.containsKey(key)) {
            fail(String.format("The following key could not be found: %s", key));
        }

        return bundle.getString(key);
    }