If you speak Nepali language only and don't belong to any ethnicity with own language then you should not take it negatively if people with their own ethnic language speak their own language when they meet up. 

Even in the US when Nepalis meet with other Nepalis, they tend to speak in Nepali even if there are other Americans in the room.

If you have done that occassionally, then you shouldn't have issues with people of different ethnicity speaking their own language!

huh?

ok.

Chheu na tuppa kura garne ustai bujhne ustai

हैन ब्रो को हो त्यो छेउ त पुच्छरको कुरा गर्ने ?
धागो उन्न्ने कि बुन्ने चाँही लाई भनेको हो ? 

तिनिहरु सबै कालेज जाने ठिटाहरु हुन , अङ्रेजको काँन्छो छोरा झै बैंकयाने र लन्क्याउने गरेर अरुलाई राम्रो स्कुल जा भनी सकेका छन। अब पाउने भो गुन्द्रुके झतिङरे जबाफ stiffer bro .

Stiffler bro maile ta bhanne manche ko nick chai bhanne manche le bhaneko kurale justify garcha bhanya matra ho hai, nabuje maf pam

bujhne lai soonko maala. ani nabujhne lai santwona puraskar and loads of empathy

haahahahaha grgdai le hasayo....its totally different when its between Americans and Nepali people....Its kinda rude to speak Nepali with a Nepali friend  constantly when there are AMERICAN Friends who are trying to have a conversation.....lets say while sitting in the same table in a restaurant having dinner, or at work or a party its kinda awkward . BUT its also rude when two Newar friends are constantly speaking Newari while hanging out together in a group of mixed ethnicity.........because most Newars know Nepali...its also very awkward...I dont mind when that happens LOL.. .its not just with us Nepalese...Ive felt awkward when that happens while hanging out with other countries friends such as  Japanese, chinese, Brazilian, spanish friends....they start talking and laughing and other friends are left clueless............Man its AWKWARD!!!!

When i am within all Nepali friends circle i prefer Nepali language ...there were few moments where few wannabe's american nepali were showing off American language well we just kept on replying and talking on Nepali even they continued with their show off atlast they said sorry and left the group . I do respect others & everyone but when we are with our own friends ,brothers,families atleast i love talking in Nepali ...but remember onething when there are few others from other countries in your group give them a respect talk in english so everyone knows whats going on if you try to act as dumb you can talk in Nepali cause they will feel offended cause we often talk and laugh around crack few jokes but those friends might think we were just making fun of them .

General curtesy is that if you are speaking your language in a group of  ppl where thesr is a presence of ppl who dont understand your language you first ask/inform other guys that you are going to talk with your friends in your language. If you dont do that it is offensive as nepalinetwork says.
and even after doing that,  you can speak in your language for only short period of time. If your conversation is longer then again it is offensive.

 I speak in Nepali, with some minimal smatter of English, when I am in the company of exclusively Nepalis.  If another Nepali speaks to me in English, I reply in Nepali.  Most of the time, that person gets the hint.

If there is a non-Nepali in the circle, I speak mostly English.  When I or someone says something in Nepali, I immediately translate it into English so that that non-Nepali person does not feel left out.  I think doing otherwise is rude.  

I believe this is a common courtesy among civilized people, though I have found people who do not have such sense, be it Newars talking in Newari whilst non-Newars are part of the company or Japanese talking in Japanese when there are non-Japanese around.