Almost a Problem...
When I hear that "almost my friends are going to Hokkaido," assuming that the error is with the use of 'almost,' I'm faced with the dilemma of whether she means that most of her friends are going, though some aren't, or that they'll spend their time mostly in Hokkaido, but will also go to some other places. What should have been 'most of' or 'mostly' came out as 'almost,' and left me up in the air.
In the case of errors like "Almost Americans are fat," the intended meaning is not elusive -- the target is no doubt "Most Americans are fat." "Americans are mostly fat," while grammatically possible, is not so likely: the human body is mostly water. If I hear that "the water is mostly hot enough for tea," which should be 'almost hot enough,' it doesn't confuse me so much as conjure up a humorous image -- I find myself imagining little pieces of hot-enough water floating among some not-yet-hot-enough ones.
Historically, both "almost" and 'most' date back nearly 1,000 years in English, with 'almost' being formed as a compound of all+most. There was apparently no word 'mostly' until late in the 16th century, and until that time the meaning of 'almost' included the idea 'mostly all, nearly all' which is quite close to the way it's often used in error by non-native speakers today. With the development of "mostly," that sense correspondingly disappeared from 'almost,' leaving it with only the slightly negative implication of "very nearly," "all but," "a little less than (completely)."
'Almost' seems to correspond in meaning and to a large extent in usage, to the Japanese [hotondo], and this is probably the root of the problem for Japanese speakers of English: ideas which are expressed with [hotondo] may appear as 'most (of)', 'almost' or 'mostly' in English, a one-to-three (or four) correspondence bound to result in confusion.
Among these three "confusables," the use of 'most (of)' seems the least likely to cause trouble, being so close to [hotondo no], though in fact less experienced speakers may be bothered by when to use the 'of.' It only appears when the modified noun is preceded by 'the, these, those, a possessive form, or before the pronoun 'them.' So, "most boys," or "most of the boys," but never "most of boys" or "most the boys."
Considering the usage of 'almost,' it appears as the first choice for numerical expressions like "That lesson cost me almost a hundred dollars," "They first settled here almost 75 years ago," or "Almost 1000 people showed up at the opening." These suggest a basic idea of modifying something complete, perfect, 100%, to make it less so. So when 'almost' is used with words like "finished, done, dressed, built, written" etc., or "ready, full, empty, dead..." it adds that meaning of "not quite, nearly." "Almost late" isn't late, and "almost empty" isn't empty. So, "I was almost asleep," "the vacation is almost over," or the old song title, "It's almost like being in love." 'Almost' appears frequently with time expressions like "I left there almost three hours ago," "We'd better leave; it's almost morning," or "I can't believe it's the 20th -- it's almost Christmas already!" with the similar sense of 'not yet.'
With 'mostly,' the historical 'newcomer' of the group, the idea of 'for the most part' shows up when it is used for expressing quantities without numbers, especially when in some sense they can be counted, like "I don't know why they call this beef stew -- it's mostly potatoes," or "The students in that school are mostly Chinese." English and Japanese seem to part ways here. "I'm mostly at home on weekends" might be a likely candidate for [hotondo] in Japanese, but it's not a case where "almost" could be used in English, unless it were as "almost always." In "That garden is mostly weeds," or "Those boys are mostly from the neighboring town," English isn't aiming for the "less-than complete" idea, and so "almost" doesn't fit.
It seems that the English 'almost' emphasizes 'less-than-ness' while the Japanese [hotondo] focuses on 'mostly-ness.' Of course these are two sides of the same coin, but there are clearly some times when they don't match well enough for 'translation' English to succeed. Like most areas of language fluency, it's an area where we have to strive to somehow get in touch with the spirit of the language, and put the dictionaries aside.