Language selection should be appended to the prompt suffix, not replace it

[amagnolo]

Describe the bug
agent_prompt_suffix hook should append to the existing suffix rather than replacing it.

To Reproduce
Steps to reproduce the behavior:

1. activate the plugin
2. prompt suffix is replaced rather than enhanced with language selection

Expected behavior
Language selection should be added to the existing suffix.

Additional context
Current implementation conflicts with other plugins like https://github.com/pieroit/haloperidol.

Workaround/fix
In fast_setup.py remove suffix = assignment, leave remaining suffix += concatenations.