Paragraph style definitions have an attribute called Next Style in the
Organizer tab. If you set it to a specific style (default is same style as the defined one), paragraph style will automatically switch when you hit the return key.
To achieve what you are looking for, define two custom paragraph styles derived from Text Body (Corps de texte if using the French version), e.g. English text and French text.
In English text,
Fonts tab, set Language to English from the drop-down menu; in
Organizer tab, set Next Style to French Text (this is possible only after style creation; you’ll then need to visit English text twice).
Do the corresponding procedure for French text.
The reason I recommend to create two new paragraph styles is to keep the possibility to write “ordinary” paragraphs with Text Body without the flip-flop effect.
Assign the adequate paragraph style to your first language-specific paragraph and the switch occurs automatically. You can break it by manually setting paragraph style to another one.
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