How can you edit the Accent Menu? The English language doesn't really include any accented words but other languages do.
Yet I like to write using accents and diacritics, as needed in each language (Romanian, French, German, etc.).

Unfortunately Mac OS X has never really made it easy to accent a letter before. Lion has simplified it, now accenting a letter is easy thanks to the Accent Menu -- example:



Still, if you take Romanian as an example, most of the specific diacritics are not reachable via the Accent Menu:


a » ă
s » ș
t » ț


Is there a way to edit the Accent Menu to add the accents that are used for a specific language?
This menu does include the Romanian diacritics when you're using the Romanian input source. I've provided a tutorial on quickly changing the input sources in:

How can you customize the keyboard layout / shortcuts?