This part of the expression “いろいろな くに の ひと と はなし” means “Various people and conversation from countries”?
And in the part “を する たのし さ を しり ました。”, what is the verb and the corresponding complement of each “を” particle?
“いろいろな くに の ひと と” is “with people from various countries”, and “はなし を する” is “talk” or “talk with”. So “いろいろな くに の ひと と はなし を する” means “talk with peopl from various countries”. Cutting the phrase to “いろいろな くに の ひと と はなし” doesn’t seem right.
You can think of it like the following diagram. In Japanese, the earlier phrase makes a sub-subclause beloning to the first word on the next level.
いろいろな くに の…from Various Countries
…ひと と はなし を する…of Talking With People
…たのしさ を…The Pleasure
…しりました…I Got To Know
So the first を works as: はなし を する (V: する、 O: はなし, “Do the talking” literally).
The second を: たのしさ を しりました (V: しりました, O: たのしさ, Knew the Pleasure).
I think the only connection that might not be immediately clear is the “…する たのしさ…” part. This is similar to the English infinitive as in “pleasure to…”, that is, “…する” is functioning like the “…to…”. For exmple “Pleasure to eat” (pleasure of eating") would be "たべる たのしさ” - the root form する、たべる, etc. works like the English infinitive in such phrases.