How to check in which language the SharePoint was installed?

First and easy set is. Go to sharepoint central admin 15 hive folder (For SharePoint 2013), 14 for 2010, 12 for 2007.

2007

c:\program files\common files\microsoft shared\web server extensions\12\resources

2010

c:\program files\common files\microsoft shared\web server extensions\14\resources

2013

c:\program files\common files\microsoft shared\web server extensions\15\resources

You can see all folder with numbers..
If you see 1033 mean it is English language has installed.It would be en-us, en-uk.. check resource file you will find which english.
1036- French - France (fr-fr)
1031-German - Germany(de-de)
1034-Spanish - Spain(es-es)
1040-Italian - Italy(it-it)
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Ref:https://msdn.microsoft.com/en-us/goglobal/bb964664.aspx

Another way is you can check the registry also..