The sort order in the language dropdown in Sitecore 9 Forms Builder seems pretty random, as seen in the below screenshot. This is rather confusing when having a lot of languages (in this case the customer has 95 languages). Therefore, I will show you how to make a custom sort order of the list.

The data in the list is fetched from a pipeline called forms.getLanguages (located in App_Config\Sitecore\ExperienceForms\Sitecore.ExperienceForms.Pipelines.Client.config). We can sort the list by extending the pipeline with our own processor handling the sorting. Below is an example that sorts the list using Sitecores own LanguageComparer class. This class sorts the languages so that it matches the sort order of the language definition items located under /sitecore/system/Languages, but you can make your own order, e.g. by display name or item name.

public class SortLanguagesProcessor : MvcPipelineProcessor<GetLanguagesEventArgs>
{
    public override void Process(GetLanguagesEventArgs args)
    {
        var sortedLanguageNames = args.ItemLanguages
            .OrderBy(item => item, new LanguageComparer(args.FormBuilderContext.Database))
            .Select(item => item.Name)
            .ToList();
        args.Languages = args.Languages.OrderBy(lang => sortedLanguageNames.IndexOf(lang.Name));
    }
}

Note that the pipeline arguments contains two lists of languages: a list of Language objects and another list of LanguageListItem objects. The latter is the one you want to sort, as this is the data sent to the view. Last but not least, don't forget to add your processor to the config file.

This is the result of my sorting implementation, where the order follows the same order I have sorted the language definition items.

If you want to make a custom sorting of the list in the content editor, use this post: https://xtremdev.com/2017/10/20/sort-language-list-and-show-language-flags/

And for the experience editor, use this post and implement the sorting in the ExtendedLanguageGallery.cs class: http://www.xcentium.com/blog/2016/08/05/language-flags-not-displayed