SitecoreLocalizer.cs

public interface ISitecoreLocalizer : IStringLocalizer
{
    Task Reload();
}

public class SitecoreLocalizer : ISitecoreLocalizer
{
    private static readonly Dictionary<string, Dictionary<string, string>> _sitecoreDictionary = new Dictionary<string, Dictionary<string, string>>();
    private readonly HttpClient _httpClient;
    private readonly SitecoreLocalizerOptions _sitecoreLocalizerOptions;
    private readonly IHttpContextAccessor _httpContextAccessor;

    public SitecoreLocalizer(
        IHttpClientFactory httpClientFactory,
        IOptions<SitecoreLocalizerOptions> sitecoreLocalizerOptions,
        IHttpContextAccessor httpContextAccessor)
    {
        _httpClient = httpClientFactory.CreateClient("sitecoreLocalizer");
        _sitecoreLocalizerOptions = sitecoreLocalizerOptions?.Value;
        _httpContextAccessor = httpContextAccessor;
    }

    public LocalizedString this[string name]
        => new LocalizedString(name, _sitecoreDictionary[_httpContextAccessor.HttpContext.Features.Get<IRequestCultureFeature>().RequestCulture.Culture.ToString()][name]);

    public LocalizedString this[string name, params object[] arguments]
        => new LocalizedString(name, string.Format(_sitecoreDictionary[_httpContextAccessor.HttpContext.Features.Get<IRequestCultureFeature>().RequestCulture.Culture.ToString()][name], arguments));

    public IEnumerable<LocalizedString> GetAllStrings(bool includeParentCultures)
    {
        foreach (var dictionaryItem in _sitecoreDictionary[_httpContextAccessor.HttpContext.Features.Get<IRequestCultureFeature>().RequestCulture.Culture.ToString()])
        {
            yield return new LocalizedString(dictionaryItem.Key, dictionaryItem.Value);
        }
    }

    public async Task Reload()
    {
        foreach (var culture in _sitecoreLocalizerOptions.Cultures)
        {
            var response = await _httpClient.GetAsync(_httpClient.BaseAddress.AbsoluteUri.Replace("[language]", culture.ToString()));
            if (response.IsSuccessStatusCode)
            {
                var content = await response.Content.ReadAsStringAsync();
                _sitecoreDictionary[culture.ToString()] = JsonConvert.DeserializeObject<DictionaryModel>(content).Phrases;
            }
        }
    }

    public IStringLocalizer WithCulture(CultureInfo culture)
    {
        throw new System.NotImplementedException();
    }
}

public class DictionaryModel
{
    public Dictionary<string, string> Phrases { get; set; }
}

public class SitecoreLocalizerOptions
{
    public IEnumerable<CultureInfo> Cultures { get; set; }
}