On the plugin single page (Gutenberg plugin test page), there are styles removing the outline from the languages button, which cause it to not have a focus outline when receiving keyboard focus.

This is the element:

This is the CSS causing interference:

.button-link {
    background: none;
    border: 0;
    border-radius: 0;
    box-shadow: none;
    cursor: pointer;
    margin: 0;
    outline: none;
    padding: 0;
}

Here's a quick video showing the missing outline on focus.