I have seen several different articles, and I want to know, to be sure what is right,
How add meta og:locale
example: en_US or en-US
Which is correct? or is the same?
on my website I have add it like this
<meta property="og:locale" content="en_US"/>
According to their documentation here http://ogp.me/,
og:locale - The locale these tags are marked up in. Of the format language_TERRITORY. Default is en_US.
The accepted format is en_US, so your meta property tag is in the correct format. If you want to add an alternative language, you can do so with
<meta property="og:locale:alternate" content="fr_FR" />
<meta property="og:locale:alternate" content="es_ES" />
As stated in their website.