Parsoid currently has this in its CSS,
// Default to right alignment. This is needed since Parsoid only specifies the // alignment class when the alignment is explicitly set. margin: 0.5em 0 1.3em 1.4em; clear: right; float: right;
which will flip, presumably if the UI flips.
However, in the legacy parser output, we have,
# Create a thumbnail. Alignment depends on the writing direction of
# the page content language (right-aligned for LTR languages,
# left-aligned for RTL languages)
# If a thumbnail width has not been provided, it is set
# to the default user option as specified in Language*.php
if ( $frameParams['align'] == '' ) {
$frameParams['align'] = $parser->getTargetLanguage()->alignEnd();
}from https://github.com/wikimedia/mediawiki/blob/master/includes/Linker.php#L386-L393
which, as stated, depends on the page content language, not the UI.