For that area, the weather *should* be fairly decent - temperatures in the 60's, possibly the low 70's, but always bring an umbrella as you never know when it will rain/drizzle or shower. Of the places you mention, Paris probably will have the best weather. I've been to all the countries and cities you mention, including in June, and have had glorious weather. But that's not always the case. Just be prepared.
I would make the same weather comment as I make for Alaska or trans-ocean crossings. The weather is unpredictable.