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This is a pretty ugly hack, but maybe it will inspire you to something better.

leastCJK = ToCharacterCode["⺀"][[1]];
StringMatchQ[#, _?(ToCharacterCode[#][[1]] < leastCJK &)] & /@ 
  {"a", "1", ".", " ", "中", "あ"}

{True, True, True, True, False, False}

Note: "⺀" is unicode character U+2E80, CJK RADICAL REPEAT

This hack can be with Repeat to ignore CJK strings

StringCases[
  "the quick fox...本当に日本語を勉強していますか...jumped", 
  Repeated[_?(ToCharacterCode[#][[1]] < leastCJK &)], 
  Overlaps -> False]

{"the quick fox...", "...jumped"}

This is a pretty ugly hack, but maybe it will inspire you to something better.

leastCJK = ToCharacterCode["⺀"][[1]];
StringMatchQ[#, _?(ToCharacterCode[#][[1]] < leastCJK &)] & /@ 
  {"a", "1", ".", " ", "中", "あ"}

{True, True, True, True, False, False}

Note: "⺀" is unicode character U+2E80, CJK RADICAL REPEAT

This hack can be used with Repeat to ignore CJK strings

StringCases[
  "the quick fox...本当に日本語を勉強していますか...jumped", 
  Repeated[_?(ToCharacterCode[#][[1]] < leastCJK &)], 
  Overlaps -> False]

{"the quick fox...", "...jumped"}

This is a pretty ugly hack, but maybe it will inspire you to something better.

leastCJK = ToCharacterCode["⺀"][[1]];
StringMatchQ[#, _?(ToCharacterCode[#][[1]] < leastCJK &)] & /@ 
  {"a", "1", ".", " ", "中", "あ"}

{True, True, True, True, False, False}

Note: "⺀" is unicode character U+2E80, CJK RADICAL REPEAT

This hack can be with Repeat to ignore CJK strings

StringCases[
  "the quick fox...んは本当に日本語を勉強していますか...jumped", 
  Repeated[_?(ToCharacterCode[#][[1]] < leastCJK &)], 
  Overlaps -> False]

{"the quick fox...", "...jumped"}

This is a pretty ugly hack, but maybe it will inspire you to something better.

leastCJK = ToCharacterCode["⺀"][[1]];
StringMatchQ[#, _?(ToCharacterCode[#][[1]] < leastCJK &)] & /@ 
  {"a", "1", ".", " ", "中", "あ"}

{True, True, True, True, False, False}

Note: "⺀" is unicode character U+2E80, CJK RADICAL REPEAT

This hack can be with Repeat to ignore CJK strings

StringCases[
  "the quick fox...本当に日本語を勉強していますか...jumped", 
  Repeated[_?(ToCharacterCode[#][[1]] < leastCJK &)], 
  Overlaps -> False]

{"the quick fox...", "...jumped"}

This is a pretty ugly hack, but maybe it will inspire you to something better.

leastCJK = ToCharacterCode["⺀"][[1]];
StringMatchQ[#, _?(ToCharacterCode[#][[1]] < leastCJK &)] & /@ 
  {"a", "1", ".", " ", "中", "あ"}

{True, True, True, True, False, False}

Note: "⺀" is unicode character U+2E80, CJK RADICAL REPEAT

This is a pretty ugly hack, but maybe it will inspire you to something better.

leastCJK = ToCharacterCode["⺀"][[1]];
StringMatchQ[#, _?(ToCharacterCode[#][[1]] < leastCJK &)] & /@ 
  {"a", "1", ".", " ", "中", "あ"}

{True, True, True, True, False, False}

Note: "⺀" is unicode character U+2E80, CJK RADICAL REPEAT

This hack can be with Repeat to ignore CJK strings

StringCases[
  "the quick fox...んは本当に日本語を勉強していますか...jumped", 
  Repeated[_?(ToCharacterCode[#][[1]] < leastCJK &)], 
  Overlaps -> False]

{"the quick fox...", "...jumped"}