The development of DBCS applications falls into two steps: function development step and translation step. As far as the function development step is concerned, no language knowledge is required. For example, even if you have little or no knowledge of Japanese, you can code a program supporting DBCS and test the DBCS functions using the DBCS alphanumerics.
However, translators need a specific language knowledge during the translation step in which messages and panels are translated into the national language. The actual translation work needs to be done by native speakers.
If you support language-unique functions such as grammar check in your program, you should also know the language and the country customs, which are beyond the scope of this book.