In plain words
Whichever form a devotee wishes to worship with faith, I make that very faith steady in that person.
Word for word
Whoever, a devotee, wishes to worship whichever form with faith, of each of them that very faith I make steady.
The commentators
Bhagavadgītābhāṣyam
तेषां च कामीनाम् —
यः यः कामी यां यां देवतातनुं श्रद्धया संयुक्तः भक्तश्च सन् अर्चितुं पूजयितुम् इच्छति, तस्य तस्य कामिनः अचलां स्थिरां श्रद्धां तामेव विदधामि स्थिरीकरोमि ॥ २१ ॥
In plain words
And of those who are ruled by desire —
Whoever is full of desire, and whichever form of a deity he wants to worship — coming with faith, coming as a devotee — in him, in each such person, that very faith I make unshakable. Unshakable meaning steady. And I ordain it, meaning: I am the one who makes it firm.
Close to the Sanskrit
And of those desirous ones —
Whichever desirer, whichever deity-form, being joined with faith and devoted, wishes to worship — that is, to honour — of him, of that desirer, unshakable, that is, steady, faith, that very faith I ordain, that is, I make firm.