=== Is it a Qt application that bundles its own Qt library? === Bundled Qt library usually uses theirs own plugin directory, which is different from system's Qt. And commonly, they are also using Qt different on system Qt, which will also make it incompatible if you simply copy the system fcitx-qt files. But anyway, you can start to check whether it loads your copied files with following environment variable. Depending on how the XIM application is written, it may need to find specific font to make it work. On Archlinux xorg-mkfontscale is required to generate correct font dir files. After install it, you'll need to restart X Server to make it work.
程序自带 Qt 库通常使用他们自己的插件目录，与系统的 Qt 不同。通常，Qt 和系统的 Qt 版本也不一样，这也将导致你直接复制 fcitx-qt 的文件过去也不能使用。但是你总是通过设置以下环境变量检查是否有加载复制的插件。