The feature "Select Content", Release note - CMS-21131 (optimizely.com), is not currently translated it seems.

One can remedy that temporarly, if not wanting to wait for an update, by adding an "override"-xml for the language you want to support to your editors. My EpiserverOverrides.xml:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<languages>
  <language name="Swedish" id="sv">
    <episerver>
      <cms>
        <!-- Saknas för stunden i senaste 12.7.0 -->
        <contentediting>
          <editors>
            <itemcollection>
              <emptyactions>
                <buttonlabel>Lägg till länk</buttonlabel>
              </emptyactions>
            </itemcollection>
          </editors>
        </contentediting>
        <widget>
          <contentselector>
            <title>Välj innehåll</title>
            <button>
              <selectimage>Välj bild</selectimage>
              <selectmedia>Välj media</selectmedia>
              <selectvideo>Välj video</selectvideo>
            </button>
          </contentselector>
          <contentselectorplugandplay>
            <text>Släpp innehåll här eller</text>
            <buttonlabel see="/episerver/cms/widget/contentselector/title"></buttonlabel>
            <dialogtitle see="/episerver/cms/widget/contentselector/title"></dialogtitle>
            <selectcontentcommand>
              <label see="/episerver/cms/widget/contentselector/title"></label>
            </selectcontentcommand>
          </contentselectorplugandplay>
        </widget>
      </cms>
    </episerver>
  </language>
</languages>

Another heads up if using interfaces for AllowedTypes

For grouping common blocks for AllowedTypes one needs to make sure that the interface used inherits from IContentData since the Select Content-feature calls GetChildren which has a generic constraint on IContentData. As an example, also applied to IHeroBlock as in image from above:

public interface IStandardBlock : IReadOnly<BlockData>, IContentData
{
}