Sebastian,
Nothing at all in any header related to hreflang (am I missing some site setting?)
Entire <head> section from the French Home page:
<!DOCTYPE html>
<html lang="fr-FR">
<head id="Head">
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<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
<meta name="REVISIT-AFTER" content="1 DAYS" />
<meta name="RATING" content="GENERAL" />
<meta name="RESOURCE-TYPE" content="DOCUMENT" />
<meta content="text/javascript" http-equiv="Content-Script-Type" />
<meta content="text/css" http-equiv="Content-Style-Type" />
<title>
Galley Design - Accueil
</title><meta id="MetaKeywords" name="KEYWORDS" content=",DotNetNuke,DNN" /><meta id="MetaGenerator" name="GENERATOR" content="DotNetNuke " /><meta id="MetaRobots" name="ROBOTS" content="INDEX, FOLLOW" /><link href="/Resources/Shared/stylesheets/dnndefault/7.0.0/default.css?cdv=133" type="text/css" rel="stylesheet"/><link href="/DesktopModules/HTML/module.css?cdv=133" type="text/css" rel="stylesheet"/><link href="/Resources/Search/SearchSkinObjectPreview.css?cdv=133" type="text/css" rel="stylesheet"/><link href="/Portals/_default/Skins/VividGraySkins/skin.css?cdv=133" type="text/css" rel="stylesheet"/><link href="/Portals/1/portal.css?cdv=133" type="text/css" rel="stylesheet"/><link href="/Portals/_default/Skins/VividGraySkins/TWMenu/TWMenu.css?cdv=133" type="text/css" rel="stylesheet"/><link href="/Portals/_default/Skins/VividGraySkins/TWMenuMobi/TWMenuMobi.css?cdv=133" type="text/css" rel="stylesheet"/><script src="/Resources/libraries/jQuery/01_09_01/jquery.js?cdv=133" type="text/javascript"></script><script src="/Resources/libraries/jQuery-UI/01_11_03/jquery-ui.js?cdv=133" type="text/javascript"></script><!--[if LT IE 7]><link id="IE6Minus" rel="stylesheet" type="text/css" href="/Portals/_default/Skins/VividGraySkins/ie6.css" /><![endif]--><!--[if IE]><link id="IE" rel="stylesheet" type="text/css" href="/Portals/_default/Skins/VividGraySkins/ie.css" /><![endif]-->
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_gaq.push(['_trackPageview']);
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var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
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<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" /></head>