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With multiple languages at work, input of diacritical letters by keyboard happens to be quite problematic. Dozens of quirky key combinations of your native layout, still giving you not even a fraction of all the diacritical variety, or switching from language to language for specific input, each defined without single, predictable manner.
Trying to deal with this mess, I once made a keyboard with a single and intuitive method for possibly all diacritical letters. The method is simply the Option key + the base letter, followed by typing diacritic's name. Therefore pressing, say, Opt + e, followed by typing acute spits an é. There's also no need to type the entire name, since the keyboard estimates it and suggests early available output. You can get the layout and view more info at http://github.com/ichosis/Interdiacritical/.
Typographers, linguists and editors — all the people who are dealing with diacritics in their daily work — would, I think, find the keyboard useful. Especially touch-typists used to type with uninterrupted flow, what the method inherently allows. But I'd rather hear what do you guys think.