Ubuntu - French Accents with US Keyboard Layout

Original Source : https://askubuntu.com/questions/333227/type-french-accents-with-us-international-keyboard

 
 Use the keyboard layout called English (international with Alt Gr dead keys). (Do this by going to System Settings > Keyboard > Layout settings, and then add the layout you want with the + button on the bottom.)
This way, the keys to input the characters ', ", `, ~, and ^ become "dead keys" while you hold pressed Alt Gr. After releasing the key combination with Alt Gr, no characters will be print (hence "dead"), but inserting a letter afterwards will yield a variation of the base character of the letter, in accordance with the dead key you pressed before; for example, é, ë, è, ẽ, and ê for e (respectively). (If you have no explicit Alt Gr key, just use the rightmost Alt key.)
So in summary, to insert a symbol

1. press and hold Alt Gr,
2. press the modifier key / key combination; e.g., `,
3. release all keys pressed so far, and
4. press the base character key; e.g., E for è.

Note: You can also enter an acute accented vowel by just pressing Alt Gr + (vowel).
(Source: this post on french Ubuntu forums.)