You ask: "I wonder if all the study subjects spoke English or the same language or why the language variable is not mentioned...."
To the authors' credit, they did not apply language barriers to their literature search. Also, they translated their summary into multiple languages.
On the other hand, they note: 'Because of language barriers, local literature in LMICs was excluded, which could have biased our results.' How they define 'local literature' is not clear. I have invited to the authors to join us.
Full text: https://www.thelancet.com/journals/lanpsy/article/PIIS2215-0366(20)30256-X/fulltext
Best wishes, Neil
Joint Coordinator, HIFA Project on Multilingualism
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HIFA profile: Neil Pakenham-Walsh is coordinator of the HIFA global health campaign (Healthcare Information For All - www.hifa.org ), a global community with more than 20,000 members in 180 countries, interacting on six global forums in four languages in collaboration with WHO. Twitter: @hifa_org FB: facebook.com/HIFAdotORG email@example.com