I am currently using Rstudio Version 1.1.419, which cannot use Chinese input method with fcitx. The official solution does not work anymore. Finally I found this blog that fixed the problem. Here is my summary on Debian stretch:
wget http://ikuya.info/tmp/fcitx-qt5-rstudio.tar.gz
tar xf fcitx-qt5-rstudio.tar.gz
cd fcitx-qt5-rstudio
sudo dpkg -i libfcitx-qt5-1-rstudio_1.0.5-1_amd64.deb
# installation of fcitx-frontend-qt5-rstudio_1.0.5-1_amd64.deb has a problem
# here we just get the library
dpkg -X fcitx-frontend-qt5-rstudio_1.0.5-1_amd64.deb extract
sudo cp extract/usr/lib/rstudio/bin/plugins/platforminputcontexts/libfcitxplatforminputcontextplugin.so /usr/lib/rstudio/bin/plugins/platforminputcontexts/
Hope this helps and hope Rstudio can solve the problem in the next version!