Match the seven names
C = do, D = ré, E = mi, F = fa, G = sol, A = la, B = si. This is the fixed-do mapping used by the French version of the app. The same sounding note keeps the same position on the guitar when you switch languages; only its name changes.
English-language resources commonly use letters. French-language resources commonly use do, ré, mi, fa, sol, la, si. Some teaching traditions use movable do, where do means the tonic of a key rather than always C. That is a different system from the fixed names used here.
Keep the accidental and octave
The accidental stays attached to the note: C♯ is do♯, and B♭ is si♭. An octave number distinguishes notes that share a name but have different heights. The app uses scientific pitch notation: middle C is C4, and A4 is 440 Hz.
For guitar, separate the written octave from the sounding octave. The low open string is E2 (mi), written E3 on the treble staff. Practise recognizing the letter and solfège name together, then use the string and fret to distinguish different octaves.
Try it yourself
Translate A, C♯ and B♭ into French note names.
Show the answer
La, do♯ and si♭. Their pitches do not change when their names are translated.