My font doesn´t work in french or german keyboard system

Hi, I have made a Tamil font, it is made on a Norwegian keyboard system. I have used letters like äëïöü, áéíóú, but when I try my font on a German or French keyboard, these letters does not work. How can I make it work? It works fine on Norwegian keyboards.

Why don’t you use the proper Tamil encoding? Then you would switch to a Tamil keyboard layout and would type the proper unicode?

Please don’t invent a new encoding. It might be easier to input but everything else will break. You can’t use existing texts (or use your texts with other fonts). Search and line layout will be all over the place (if the layout engine will try to do hyphenation and case folding.
So please use Unicode.