Currently the SCIM project has a wide range of input methods (some may need
other libraries), covering more than 30 languages, including
(Simplified/Traditional) Chinese, Japanese, Korean and many European languages.
What's more, Composing/Dead key support is one of the built-in features.
In addition, several projects have been established to design IMEngines for
SCIM and others supply their own SCIM plugins.

Homepage:
https://github.com/scim-im/scim
