Warmly welcome Russian friends to visit our school

Why use English? Isn’t it good to use Russian?

At that time, such slogans did not need to be so complicated. The most convenient, short and idiomatic statement

Добро пожаловать! (Welcome!) would be enough.

Warmly...Russian friends...our school...etc., no need to translate it.

Otherwise, it would be like this

Горячо приветствуем российских друзей, посещающих нашу школу! It's a purely Chinese style of foreign language, and you may not get used to it.