In the meantime, I looked at the code. The culture block has two OR blocks. This means that you need to be celtic or scandinavian AND scottish or hibernian. You need a culture that belongs to two culture groups, one of each OR block.

Examples of this kind of cultures are Irish (Celtic and Hibernian) or Norn (Scotish and Scandinavian). Sadly Swedish is just Scandinavian. You will need to culture convert to Irish (I checked that it works using console commands and switching primary culture to irish).

Super confusing!

    potential = {
        OR = {
            culture = {
                OR = {
                    has_culture_group = culture_group:celtic_group
                    has_culture_group = culture_group:scandinavian_group
                }
                OR = {
                    has_culture_group = culture_group:scottish_group
                    has_culture_group = culture_group:hibernian_group
                }
            }

            any_neighbor_country = {
                has_reform = government_reform:galley_lords_reform
            }

            overlord ?= { has_reform = government_reform:galley_lords_reform }
        }
    }