I downloaded the update (1.2 GB!) and spent a little time looking for the language option. I found it in Options --> Interface.
I changed to German and clicked on done. I got a blank box which I assumed should have contained text telling me to restart the game. I did restart and everything was in English with smaller text. It took another few seconds for me to realize that I still had mods enabled, and one of them modifies the UIText file, and it was overwriting the German UIText file since they probably have the same name.
I disabled mods and restarted the game. The main menu and all the Options pages were in German, as expected. I switched to French, restarted and they were all in French. I switched to Russian, restarted and they were all in Russian. It all worked correctly for me.
I then decided to load a save that was in English and made with the version of 4.1 before this update. I wanted to see which language would be used. I couldn't tell because the game crashed. A crash report was sent in a few minutes ago.
EDIT AND UPDATE: After getting back to English and making this commet, I decided to load the save mentioned in the previous paragraph into the current update. It also crashed, and a new crash report was generated. I guess saves aren't compatible with this update. END EDIT AND UPDATE.
While still using the Russian language, I restarted GC3 and started a new game. Everything I checked was in Russian.
One problem that persisted was that some places had long words/phrases that didn't fit in the space provided. I didn't check much and only noticed it in German and Russian. Also, a few places had English words where I thought a word in the chosen language should be used. I don't know enough of the other languages to be sure I'm right, so I'll leave this to those fluent in the other laguages to report.
I think this update will please everyone who wants to play the game in German, French or Russian.