You don't have to learn Japanesw seriously for a short stay. Because your effort for Japanese won't bear fruits with only for a short time. does this sound natural?
Instead of "with only for a short time", you should say "in such a short time". You do not need to separate this into two sentences. Starting a sentence with "Because" is usually a bad idea. You don't have to learn Japanese seriously for a short stay because your efforts won't bear fruit in such a short time.
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