Nice plan almostdone !
I don't know about the Adriactic, but the northern countries you listed can be pretty cold before April, and even have snow. Here's an idea of the temperature in the winter (from the Lonely Planet
Slovenia : January coldest month ; average -2 C
Croatia : sea tempature is always above 10 C, you can swim from mid-June
Bosnia and Herzegovina : it's a mountainuous country, where snowfall can last until April
Bulgaria : from December to February 3 C ; warmer around the Black Sea
Romania : average annual temperature is 11 C in the south, and 2 C in the mountains
Hungary : January around 0 C
Czech Republic/Slovakia : around freezing point in January, above 10 C in April (except in the Tatras)
Poland : possibility of sub-zero temperatures between November and March
BUT those are averages. I was in Poland-Slovakia-Hungary-Romania in late April until early June and it was nice and sunny (up to 36 C in Budapest). There were very few days where the temperature went below 20 C. In the north of Slovakia, in the High Tatras, people were still skiing in April though.
You won't need to book ahead of time a room/bed, except for Prague and Budapest if you want to stay close to downtown. Those destinations are pretty popular nowadays. I was in Budapest in April, and the only hostel I could find was a long bus ride from the center (but it was a very nice place - no regrets at all !).
In Bratislava, most hostels are closed during the off season, so search in guide books or on travelpunk's book online (link
) to find good addresses.
The language isn't a problem when you stick to major cities and tourist area, but elsewhere, you'll need patience and your grin to communicate. I personally suggest (strongly !) to learn the basics for directions and time schedule (when, where, bus, train, etc.), if you do head away from the centers.
Enough time ? It really depends on you. What kind of experience are you looking for ? You're planning 100 days in Eastern Europe, for 10 countries, so 10 days average per country. Considering that you'll spend different amount of time in each, I say it looks pretty good.
But of course, more is always better, for a thorough experience !
As far as I'm concerned, the "true Eastern Europe" lies in the countryside. So, the one thing I say is missing, is to base yourself in one town, and do several day-trips in the area. Rent a bike or a car, and get away from the city ! Northern Romania is great, around Prague, and Eastern Hungary as well. (I don't know about the ex-Yougoslavia's republics.)
Vadu Izei (north of Romania)
If the Adriatic is too cold to bath in January, try the Black Sea later in your trip.