If you load up /Views/Shared/LanguageAttributes.cshtml you'll notice that it's not pulling in any language information (in version 3.5 anyway). If not done so already create this view file in your current theme and add the following contents:
@using Nop.Core
@using Nop.Core.Infrastructure
@{
var languageSeoCode = EngineContext.Current.Resolve<IWorkContext>().WorkingLanguage.UniqueSeoCode;
if (ShouldUseRtlTheme()) {<text>lang="@languageSeoCode" xml:lang="@languageSeoCode" dir="rtl"</text>} else {<text>lang="@languageSeoCode" xml:lang="@languageSeoCode"</text>}}