def localization_for(locale, localization_key) method in localization_keys_controller can return false. That can cause undefined method 'id' for false:FalseClass error in view partial _localizations_list.html.erb:
<%- available_locales.each do |locale| %>
<%- localization = localization_for(locale, lk) %>
<tr>
<td class="localization_row" data-id="<%= localization.id%>" data-edit="<%= edit_localization_key_localization_path(lk, localization, format: :js) %>" data-editing=0 data-content="">