Indexed search always uses frontend indexing. It only varies whether any visitor calls your page or if the crawler calls the page. Each time the cache is filled (by the first call) the content gets indexed.
The language configuration can be found in the manual:
https://docs.typo3.org/typo3cms/extensions/crawler/ExtCrawler/Configuration/ConfigurationRecords/Index.html (see the field 'Configuration') or the next page where paramSets.[key]
gets described.
Be aware of any settings of the Url-parameter L
, which is used as the language parameter for TYPO3.