In Mlp_Hreflang_Header_Output::wp_head() resp. Mlp_Hreflang_Header_Output::http_header()
only the available translations are searched - excluding the current base content.
That should be included, though :
If you have multiple language versions of a URL, each language page must identify all language
versions, including itself. For example, if your site provides content in French, English, and Spanish,
the Spanish version must include a rel="alternate" hreflang="x" link for itself in addition to links to the
French and English versions. Similarly, the English and French versions must each include the same
references to the French, English, and Spanish versions.