Both "The husband is a Russian" and "The husband is Russian" are fine. But for many nationalities it is different whether you use a noun or adjective. "The husband is a Frenchman"/"The husband is French", "The husband is a Spaniard"/"The husband is Spanish", "The husband is a Swede"/"The husband is Swedish".
"The husband too is a Russian" = le mari aussi est un Russe.
This is possible but with a different meaning: the neighbor, the friend, the landlord... and the husband too... - this does not tell us that the wife is Russian.
With "aussi" at the end, the meaning is: the wife... and the husband too... - this tells us that the wife is Russian.
I believe that Carte_Blanche66's answer is correct. "The husband, too, is Russian" [in the version I was given, the punctuation was not possible] means that both husband and wife are Russian[s]; "The husband is Russian too" means as well as having some other quality [dual nationality, for example] the husband is also Russian.
When it comes no nationalities, you have nouns and adjectives:
- he is Russian (adj) = il est russe (not capitalized)
- he is a Russian (noun) = c'est un Russe (capitalized)
Note that "il est un Russe" is not correct because "he is + determiner + noun" has to translate to "c'est + determiner + noun".
Have to be a bit careful... In US English, if you say "a Russian" it can have a negative connotation. When we are in conflict with a country, saying a/an + nationality is not just a neutral statement of fact. E.g. during WWII and for awhile afterwards, saying "he is a German" would definitely be taken negatively. Even though the Cold War is over, it can still be taken negatively in referring to Russian nationals. In the US, we rather say one is "from + country of nationality" or without an article (adj form) to keep from implying a negative connotation.
Sure, but woudn't it be possible to do so with a sentence which woudn't sound ackward in French? If the aim of DL is to teach grammar first, fair enough, but as a native speaker I just thought it useful to signal (to the users more than to the DL team) that the sentence wouldn't actually be used in a conversation. I did not (just) mean to criticize DL , I mostly wanted to make a clarification about usage. Regards.