There are several points to note here.

The basic Hindi alphabet does not have symbols for certain sounds present in other languges like Urdu or English. Examples are the "ph" as in "phone", the "z" sound as in "zoo" etc.,.

In modern Hindi additional symbols have been added to the basic script to represent these sounds. To do this, the Hindi alphabet that is closest in pronounciation has been slightly modified to represent these new sounds (in the case of consonants). So for the "pha" sound, the letter फ has been chosen and a dot (called the nukta) has been added to it to make it convey the non-Hindi sound of "pha", like this फ़. Similarly we also have क़ and ज़.

In the case of vowel sounds that do not exist in Hindi, only one new symbol has been added to the basic list of Hindi vowels, this is the "o" sound that exists in words like doctor (the first "o"). For this a new vowel ऑ has been added.

So when you look at your translation of font, we have

फांट (which when back translated would read fant)


There is also an infringement of a spelling rule here. In Hindi, the anuswar, that is the dot above the word, is used to replace the pancham varna (ङ, ञ, ण, न, म) that comes at the end of each group of consonants (कवर्ग, चवर्ग, टवर्ग, तवर्ग and पवर्ग)), only when the pancham varna joins with a consonat of the same group.

For example, consider the क group and its pancham varna ङ:

क ख ग घ ङ (ka group, the last is its pancham varna)

Now consider the word ganga, which can be written as follows:

गङ्गा (Note the pancham varna ङ joining with a consonant of its own group ग)

In modern Hindi this word is written with an anuswar like this:
गंगा

In standardised Hindi, it would be considered an error to write गंगा, as गंङ्गा।

Now let us take your case of font written as फांट

The consonent group involved here is ट group and the consonent here is ण. Here is the full ट group:

ट ठ ड ढ ण

So, the फांट is equivalent to फाण्ट, for the nukta here stands for the pancham varna of the group of consonants ट, which is ण. So
the actual pronounciation of your word is फाण्ट, whereas, the English word font has a clear "न" sound and not a "ण" sound. In fact the "ण" sound exists only in Indian langauges and even here it is very rare. For example, there are no words in Hindi starting with ण.

Now, let us consider the microsoft version

फ़ॉन्ट

This backtranslates to fhont, because of the nukta and the half "na" sound joining with "ta". This closer to the pronounciation of the original word font.

Because the half na, although a pancham varna, does not belong to the consonent group of ट, it cannot be represented as an anuswar. So it would be wrong to write:
फ़ोंट

Becasue this would be equivalent to फ़ोण्ट.

Most Hindi words are phonetic in nature, and are written just as they are spoken. However, to render foreign words, correctly one has to carefully match the Hindi sounds with the foreign sounds and chose the correct vowel and consonents.

Many foreign words become cumbersome or difficult to write in the Hindi alphabet. These words are often modified to suit Hindi phonetics. They then become accepted in Hindi in the modified form. Examples are

अकादमी (academy), लालटेन (lantern), कंडील (candle), etc.

In the case of font, this kind of simplification has not yet taken place. So it would still be taken as an error to write it as फांट. May be in a few years, if a lot of people finding the फ़ोन्ट form difficult to write and pronounce start using the simplified version फांट, then that would become accepted usage in Hindi and would no longer be an error.