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This may be a little bit outside the scope of the original question, but another use for hreflang tags is when you only have a single language, but localized content in different countries. So, for example let's say you have web sites/pages in the US and Australia. Both are in English. But you differ the content for each market. In that case, you would use hreflang tags to relate a similar page/site between the two countries. "en-us" and "en-au". I realize that's not exactly what the original post asked. But adding this info to what has already been answered.