the altar is built separated from the wall, so as to be able to move around it easily and to celebrate looking towards the people, which thing is convenient to realize wherever possible
As an FYI, the part that is “convenient to realize wherever possible” is that the altar be built away from the wall.
Here is the correct interpretation from Rome
The Congregation for Divine Worship and the Discipline of the Sacraments has been asked whether the expression in n. 299 of the Institutio Generalis Missalis Romani constitutes a norm according to which the position of the priest versus absidem [facing the apse] is to be excluded. The Congregation for Divine Worship and the Discipline of the Sacraments, after mature reflection and in light of liturgical precedents, responds:
Negatively, and in accordance with the following explanation.
The explanation includes different elements which must be taken into account. First, the word expedit does not constitute a strict obligation but a suggestion that refers to the construction of the altar a pariete sejunctum (detached from the wall). It does not require, for example, that existing altars be pulled away from the wall. The phrase ubi possibile sit (where it is possible) refers to, for example, the topography of the place, the availability of space, the artistic value of the existing altar, the sensibility of the people participating in the celebrations in a particular church, etc.
Part of the problem comes from English, which considers word order to be important, while it is far less so in Latin,
The definitive legal source in Roman Catholicism is always the Latin
here is the text in Latin
Altare *maius *exstruatur *a *pariete *seiunctum, *ut *facile *circumiri *et *in *eo *celebratio *versus *populum *peragi possit, *quod *expedit *ubicumque *possibile *si
Now the commentary pause of ut facile…does not change the sentence modifier of 'quod expit…( which is useful wherever it is possible).
Think of the ‘ut’ as an interlineal comment, like we use parens today
The altar should be built apart from the wall,(so that it is possible to walk around it easily and that Mass can be celebrated at it facing the people), wherever possible.
The part that is useful wherever possible is the main clause, that the altar should be build away from the wall. The reason given is that the altar should be build away from the wall is that it can be easily walked around and a celebration toward the people can be carried out. GIRM 299 cannot be considered to state that there is a preference for the Mass to be celebrated facing towards the people, rather that the preference is that the altar be build away from the wall. And even then, as the responsum from the CDWDS indicated, that itself is not a command, but depends on local conditions.