I did this ride last year, so have a bit of hopefully useful information.
Language: I learned the total of 1 word of Finnish (notoriously difficult for anglos to learn): Kiitos (pronounced "Key-toss"), which means "thanks".
Swedish is the official second language of Finland, but more people speak English than Abbaish.
Roads: pretty good. Traffic: pretty well behaved. Find will not slow down for you, but they will invariably go around with plenty of room - regardless of what is comming ahead!
Almost all roads are numbered with 1, 2, 3 or 4 digits. Pretty much all roads are built to a uniform width (ie no shoulder), and only the primary single digit roads (and the 2 digit roads in the south) need be avoided.
Maps. Particularly difficult. My Aussie purchased atlas proved more accurate than the "Karttahuone" map I purchased on arrival. This map was bi-lingual (Finnish & Swedish) aimed at Swedish tourists, and was so inaccurate as to be useless. Avoid.
The only useful looking maps I saw in Finland were sold as a 2 volume set for 30Euros ($55) in (almost) all service stations. (White cover, blue writing, only in Finnish) They are the equivalent to the NRMA/RAC atlases and their maps were very accurate, and included town maps (very important). If I were going again, I'd get a set of these, tear out the pages I wanted/needed, then ditch (or post home) the rest.
Road signs in Finland are almost as good as Australia
... but ... town names in Finnish can be spelt differently depending on the context in which they are used. Into Sydney, Out of Sydney, through Sydney or just Sydney will all appear on a Finnish sign as one word, with a significantly different spelling. You *need* to know the route number of the road you are taking.
Accommodation: I camped everywhere. In Finland, and all the Scandinavian countries except Denmark, you are legally allowed to camp anywhere - within reason (not <100m form someone's home, not on a crop). I used 4 formal camp grounds, and the rest of the time I camped rough. There is very little public land in Finland. Almost everything is forest, and almost all of that is privately owned and subdivided up. But finding somewhere to camp is not particularly difficult.
Food: Almost ever dot on the map represents a shop of some sort. Some places it is only the shop. Most small towns have at least 2 but usually 3 small sell everything supermarkets. This was true throughout the country. I'd been warned that north of Kemijarvi I'd find nothing, but it wasn't true.
Finish bread is this black, stodgy stuff. They tell me it's really good for you, but the best I can say about it is it's doesn't look as bad as it tastes. Finish staples are meatballs with potatoes. While it might look like PAL comming out of the tin, it doesn't taste that bad IMHO. The Fin's idea of exotic food is lettuce & tomato salad
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Route: Well, I took an inland route, hoping to find more remote wilderness and avoid touristy areas. This turned out to be .. well .. a bit boring. Having seen some very impressive coast lines in my time, including having lived on one for a while, coastlines need to be pretty good to impress me, so I don't feel I missed much skipping the Baltic coast of Finland. But there *is* a decent looking route up the coast.
Terrain: Reasonably flat. There were no major climbs, and even though I had headwinds every day, averaged over 20kph every day.