In Relativity under the the Administration tab, and the Workspace details sub-tab, you'll see an option to select the SQL Full Text Language. This setting must be set to the language of the documents loaded into your database. If the documents are in multiple languages in roughly equally amounts you have a challenge. kCura recommends changing the SQL Full Text Language to the language which has the most complex stemming and word breaking rules.
How do we determine which language has the most complex word break rules? While Western languages follow similar word breaking rules, those used for German will be more complex than those used in English or Spanish. German has more compound words. See this reference on a Microsoft page regarding SQL Server. Chinese, Japanese, Korean, and Thai use a more complex word breaking system because text can be broken into individual characters, and even individual characters can be divided.