I wonder how did you manage to add duplicate Language column to your site columns since it is not possible to add them trough UI (the column reserved message will appear).
If you added your Language column directly on list then your new column will be added but it internal name will be Language0 (but by default design this column will be not added to Site columns).
When you click Language column under Site Columns you can check URL for your internal column name. It should contain querystring
field=Language. This is the only way you can be sure you are editing the right column.
When adding new columns to your site avoid using reserved names (there are some mechanisms to prevent you doing so but obviously they are not 100% working). The problem you are now experiencing is just 'the tip of the iceberg'. If you really need to use reserved name and if you are using UI to add columns do this:
- Add new column and in Column name type name you know that is not reserved (in your case name it myLanguage or something similar). This will become your column internal name.
- Edit your new column and choose some other name for it (eg. Language). Your internal name will stay myLanguage but column Display Name will change.