The Forgotten Half
As an unspoken rule, most multinationals hiring foreign staff try to hire unmarried job applicants. The general thinking is that singles cost less, are easier to keep focused on the job (especially if long hours are involved), and are easier to reassign as the business requires. Having an unhappy wife, husband or kids that canﾕt adapt to Japanese culture can certainly be a huge challenge and distraction for a new employee.
In my opinion, the biggest challenge for a foreign family with children is finding an English-speaking school. I'm not talking about just paying $20,000+/year in school fees for a 5-year old, but rather simply about actually finding a school that has vacancies. It seems that although many companies are firing their foreign staff, others are going on a hiring binge. So the net result of this is that there is a shortage of places for kids at bilingual/English language schools.
The second most common challenge I've seen is with foreign spouses not being able to adapt to life in Japan. Of course, if the job package is rich enough, the employee and spouse may be able to seek refuge in the Tokyo American Club or similar communities. However, for most foreigners in Japan, such luxuries are out of the question.
Back in the early 90's I used to employ a lot of Indian engineers. Usually they would come as singles, and then within a year or so they would return home to fetch a bride. At first I thought that getting married would be great for my employees' comfort in Japan - being alone as a new arrival in Japan isn't much fun. However, the downside, as I soon discovered, was that the spouse would instead get lonely after sitting in an empty apartment for 12 hours a day (or more depending on the commute). In several cases, the spouses threatened to leave Japan and challenged their husbands to follow them back to India. Luckily none of them ever carried out the threat, but it soon inspired my personnel managers to offer a new benefit to our employees: "spousal loneliness management".
We found the best remedy to be getting the spouse involved in their neighborhood society and language training. We'd start with a neighborhood "outing," which was a bit traumatic for new arrivals. This consisted of a Japanese employee going over to the new foreign employee's house and together going with the couple to greet neighbors and invite them over for some Indian food. We found that food and communication are really excellent ways to break down the neighbors, and in a number of cases, the spouses found themselves so popular they had trouble finding time for themselves.
Language, of course, is the other answer. And at risk of beating the same drum too much, a homebound spouse who can speak Japanese can be a really excellent social asset for the couple, especially for weekend shopping trips, visits to onsen, etc. Being the Japanese-language speaker of the family helps the spouse build self-esteem and have the confidence to show visiting friends around. I recall one particular employee's wife who was extremely unhappy for about a year after arriving. She threatened to go home several times. However, she eventually knuckled-under and learned Japanese fairly well over that period, and became a star attraction of the neighborhood. Some of her friendships were so close that after the family was transferred to the USA at the end of 5 years in Japan, she cried at the thought of having to leave her hard-won friends behind!