I came across this error in nopCommerce 3.5 whereby language attributes were not being loaded and appended to the <html> tag of each page. This is especially important when running a multi-lingual store.

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>}}