I've been practicing for more than 40 years, and, in that time, I've had Teachers who said you must practice in Tibetan, others who said it was OK to practice in English, others who routinely follow all or most Tibetan with English during group practices, others who sometimes practice in Tibetan and sometimes in English during group practices, others who have said do what works for you, and others who took the "don't ask; don't tell" approach. Personally, I found the "only in Tibetan" approach did not work very well for me. I became a parrot, reading very, very fast but not really understanding or paying attention to the meaning of what I was reading. I can still say many, many Tibetan prayers and sadhanas in Tibetan and this often earns kudos from Tibetans, but, when practicing at home, I use the following methodology: If something is terma, I always say it first in Tibetan and then in English. If something is not terma, I only say it in English. This approach seems to work well for me.
Currently, I attend a local center on the 10th and 25th each month to do tshog. It's virtually all in Tibetan, often said quite fast. Since I have long history with saying Rigdzin Dupa and Yumka Dechen Gyalmo in Tibetan, I'm quite fine with this since I pretty much know all the words and their meaning in any case. However, I'm not at all sure about some of the other students, and my impression is that this is a limiting factor for the growth of this center.
I think it's important for all of us Inji practitioners to recognize that we are a transitional generation and things are not perfect. (Are they ever?) Eventually, I hope Americans at least practice entirely in English (Spanish, Chinese, Vietnamese), but this is going to take time to sort this all out. But hey, we have lifetimes; so no problem.
Just my two cents.
Pema Chophel པདྨ་ཆོས་འཕེལ