The language tag tells Google and other search engines about the language of your blog . Google's webmaster suggests that you should add a language tag to your blog so that the search engines can use your blog's hreflang tag to give search results to users searching in the language you specify.
Here's how you can add a hreflang tag to your blogspot to specify the language of your blog to Google and any other search engines .
Add the following hreflang tag in the header section of your blog/site's template editor, i.e bellow <head> tag and above the closing head tag <head/>
<link rel="alternate" href="http://example.com" hreflang="en" />
Replace "example.com" with the url of your blog and "en" hreflang tag with the language you want to use. Here are some of the language tags that you can use for other languages.
Additionally if your blog is targeting a specific region, e.g united states ,then you can specify the region also in your hreflang tags by modifying it as shown bellow
<link rel="alternate" href="http://example.com" hreflang="en-us" />
us in the above code after the dash (-) represents united states .
Here is a list of region codes on wikipedia.
If your site is multilingual then you can use the following tag in the above code