Not necessarily…

While most of the time when speaking Japanese you should try to be humble and polite, to ignore the importance of the other extreme (arguing) would be short-sighted. Unfortunately knowing how to argue with somebody in a foreign language is a required skill.

One thing that pushes many foreigner’s buttons in Japan is when (as well meaning as it may be) Japanese people assume we don’t speak Japanese because we are foreigners and speak to us first in English. I imagine this could get quite old if you are a foreigner who doesn’t happen to speak English or a non-native speaker. We might say something like this:

必ずしも外人だから、英語を話せるわけではないっすよ！
かならずしも　がいじん　だから、　えいご　を　はなせない　わけではない　っすよ！
karanazushimo gaijin dakara, eigo wo hakasenai wakedewanai ssuyo!

In English:
Just because I am a foreigner, does not necessarily mean I speak English!

The important parts of the sentence are bolded above.

必ずしも　かならずしも　kanarazushimo is an adverb meaning not necessarily.
わけではない wakedehanai is a phrase that means ‘does not mean that’.

Combine the two and we have our desired effect. To use this point you can simply put any sentence in-between 必ずしも and わけではない。

Of course it can also be used in less angry sounding sentences.
必ずしもビールが好きだから毎日飲みたいわけではない。
かならずしも　ビール　が　すき　だから　まいちに　のみたい　わけではない。
kanarazushimo biiru ga suki dakara mainichi nomitai wakedehanai

Just because I like beer does not necessarily mean I want to drink it everyday.