CellHTML lets you edit the contents of a cell and write it to the cell as HTML with the non-ASCII character in the text replaced by its HTML equivalent.

Features

If you are an ordinary user who just wants to use the Google Spreadsheets editor, press cmd + / (slash) to edit the cell in rich text, then press cmd + / or click the "Submit" button to write the changes to the cell.

If you want to use the editor with CellHTML, you need to install the CellHTML extension.

Now you can use a rich text editor on Google Spreadsheets.