长期共存，互相监督，肝胆相照，荣辱与共。

Long-term coexistence, mutual supervision, treating each other with all sincerity and sharing weal and woe.

长期共存的思想已经存在很久了。

The idea of long-term coexistence had been there for a long time.

是两种语言能够以一种稳定的趋势长期共存？

Can two languages co-exist in a stable fashion over a long period of time?