What language should I address a letter? [closed]

I would say that the international bit of the address (i.e. the country that you want to get to) should be in the language of the country you are sending from (so in this case, English), and the domestic bit (the actual street address) in the language of the country you are sending to (so Russian). Or put both.

So a reversed example (from Russia to the UK) would be something like:

Mr J Smith,
1 Travel Street
StackExchangeTown
Internetshire
??????????????

The Russian postal workers would then see "??????????????" and know it needed to go to the UK, and on arrival the Royal Mail would see the English street address.

I've done this sending postcards from Poland (with "Wielka Brytania" at the bottom) and they have got through...

• Prepend the postal code with the ISO 3166 country code, connected by a hyphen. So if you're sending a postcard to SW1A 2AA, address it at GB-SW1A 2AA (This is apparently no longer recommended; see, for example, Deutsche Post. Thanks to Tor-Einar Jarnbjos comment for this info)
• Be aware that the official UPU language is French, not English.

Traditionally, prepending with country codes was common. From Wikipedia List of postal codes:

The use of country codes in conjunction with postal codes started as a recommendation from CEPT (European Conference of Postal and Telecommunications Administrations) in the 1960s. In the original CEPT-recommendation the distinguishing signs of motor vehicles in international traffic ("car codes") were placed before the postal code, and separated from it by a "-" (dash). Codes were only used on international mail and were hardly ever used internally in each country.

Since the late 1980s, however, a number of postal administrations have changed the recommended codes to the two-letter country codes of ISO 3166. This would allow a universal, standardized code set to be used, and bring it in line with country codes used elsewhere in the UPU (Universal Postal Union). Attempts were also made (without success) to make this part of the official address guidelines of the UPU. Recently introduced postal code systems where the UPU has been involved have included the ISO 3166 country code as an integral part of the postal code.

At present there are no universal guidelines as to which code set to use, and recommendations vary from country to country. In some cases, the applied country code will differ according to recommendations of the sender's postal administration. UPU recommends that the country name always be included as the last line of the address.

I usually take a belts and braces solution, but should now reconsider the country code in light of the fact this is no longer universally recommended:

• Write the domestic part of destination address in the language of the destination. London, not Londres.
• Prepend ISO 3166 country code to postal code, as recommended by CEPT and others, but others including Deutsche Post recommend against it.
• Write destination country in several languages: language where I ship it from, in addition to French, which is the language of the International Postal Union. I usually add English as well. So shipping from Finland to The Netherlands, I would prepend the postal code with NL-, then write on the last line Alankomaat / Pays-Bas / The Netherlands.

With those, any competent postal worker should be able to figure out where it goes.

Back in the days, when you couldn't just post a selfie on a social network, so that sending postcards was way more common (not to say that it was the only option), all postcard we received at my parents home in Italy where addressed in Italian, and no post office ever complained nor anything went ever lost.

In modern times, instead...the same. I still receive (rare) postcards with the address fully written in Italian, and I receive packages (like from HK, Germany, Uk...) where the sender has put the address in full Italian.

So, basically, the answer from my personal experience is use Italian, it never failed me use the destination country language and names. Especially the names.