There is no to add a French Input, just change you input.
Here we go.
First step, click plus button to add a input source:

Secondly, fellow the steps:
English → Select ABC-Extend → Click Add , then done.
Well done! Let’s check and verify it.You can see your input status on the menu bar, just like this:

OK, to make sure you chose the
ABC-Extend , then we type words by fellow key code:plain text_snippet
Plain Text · 24 lines|
| option + e + e = é |
| option + ` + a = à |
| option + ` + a = è |
| option + ` + a = ù |
| option + 6 + a = â |
| option + 6 + e = ê |
| option + 6 + i = î |
| option + 6 + o = ô |
| option + 6 + u = û |
| option + u + a = ä |
| option + u + e = ë |
| option + u + i = ï |
| option + u + u = ü |
| option + u + y = ÿ |
| option + c + c = ç |
| option + q = œ |
| option + | = « |
| shift + option + | = » |
That’s all ! We don’t need to change ABC Input anymore.
Reference:
✦ 卷终 · 柯航存卷 ✦