ahH my japanese is still not that great but aS A SUPER QUICK/BASIC translation for those interested-
the two lines at the top are basically warning that it's still in beta stage, so there will be bugs and such; and that if you want to use a more stable version to please not download the beta version
and as for the two downloads, it reads:
painttool SAI download (full set) (Ver.1.2.0-Beta.6, installer, 2013-12-10, 2.4M)
painttool SAI download (overwrite update) (Ver.1.2.0-Beta.6, インストーラ形式, 2013-12-10, 1.2M)
so for those who want to download the full version 1.2 it's the first link, and for those who just want to update their current version (which will overwrite the earlier version) it's the second link!
and for the "インストールに関する注意点" part it's the "installation notes" - the main thing it talks about is that brushes and palettes won't be carried over into the newer version. the instructions read that you should: 1) install ver 1.2 in the folder where you have 1.1 installed, then 2) copy all the folders in ver 1.1 along with the *.conf file into the ver. 1.2 folder!
and another important note is: if you install painttool SAI under Program Files there will be various issues, so for the time being don't install under Program Files.
this is just the first couple lines though, so it might be better to wait for the full translation if you want to be careful i guess?? ; v ; hope this helps nonetheless!