as far as we know the Indonesian language ranked top 10 based on the speakers alone (https://www.wikiwand.com/en/List_of_languages_by_total_number_of_speakers). Is it possible for LanguageTool to support Indonesian as well?
Everything is possible as long as someone is willing to do it. Asking it here, however, is next to useless. The number of speakers does not matter, the number of maintainers does. Consider becoming one.