Get all dictionary items for a language on a site and spit them out in an API Controller.

DictionaryController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Umbraco.Web;
using Umbraco.Web.WebApi;

namespace Olympic.Web.Controllers.Api
{
    public class DictionaryController : UmbracoApiController
    {
        private readonly UmbracoHelper _umbracoHelper = new UmbracoHelper(UmbracoContext.Current);
        public IEnumerable<DictionaryItem> GetDictionaryItems(string culture = "en-US")
        {
            //For now, we're only going to get the English items

            var dictionary = new List<DictionaryItem>();

            var root = ApplicationContext.Services.LocalizationService.GetRootDictionaryItems();

            if (!root.Any())
            {
                return Enumerable.Empty<DictionaryItem>();
            }

            foreach (var item in root)
            {
                dictionary.Add(new DictionaryItem
                {
                    Key = item.ItemKey,
                    Value = item.Translations.Where(x => x.Language.CultureName == culture).Select(x => x.Value).FirstOrDefault()
                });

                dictionary.AddRange(LoopThroughDictionaryChildren(item.Key, culture));
            }

            return dictionary;
        }

        private IEnumerable<DictionaryItem> LoopThroughDictionaryChildren(Guid key, string culture)
        {
            var dictionary = new List<DictionaryItem>();
            var items = Services.LocalizationService.GetDictionaryItemChildren(key);

            if (!items.Any())
            {
                return Enumerable.Empty<DictionaryItem>();
            }

            foreach (var subItem in items)
            {
                dictionary.Add(new DictionaryItem
                {
                    Key = subItem.ItemKey,
                    Value = subItem.Translations.Where(x => x.Language.CultureName == culture).Select(x => x.Value).FirstOrDefault()
                });

                dictionary.AddRange(LoopThroughDictionaryChildren(subItem.Key, culture));
            }

            return dictionary;
        }
    }

    public class DictionaryItem
    {
        public string Key { get; set; }

        public string Value { get; set; }
    }
}

Thank you for sharing!
There is a small error however. When you compare til string "culture" with "Language.CultureName". The correct comparison is using "Language.CultureInfo.ToString()".