UX fixes for the language draggable cell

This should bring things closer to the recommended UX guidelines.

It changes the way the checkbox / label play together,
and changes the "localization missing" warning icon to a string.
Also some cleanup of hard-coded styles and sizes.

Bug: 26758865
Bug: 26707846
Change-Id: I07a78a1e4a41122b91e895e363a217d3e9cacd40