What do we mean when we say: Can you think in another language ? What we are talking about is, does the cognition in your head happen in another language ? I mean, when you are thinking about what to buy next from the shops, or what to do later on this afternoon, do you think about it directly in another language or does it happen in your mother tongue: i.e. you don’t think about it in another language.
Lots of my students have reported being able to think in another language after some time in the country. Few have been able to report thinking in another language whilst being in France. There’s too much French around. Turn on the radio, it’s in French, turn on the tv, it’s in French, nip down to the local baker’s, you buy your baguette in French.
However, I believe it is possible for cognition to happen in English, for example, whilst not being in the country but there is a necessary condition for that: repeated exposure to the language, and, probably, repeated acid-testing of this capacity to speak, and as a prerequesite to natural speaking, naturally thinking in that language.
Make no bones about it*, this process takes time. How much time ? 1-3 years or more, providing that the conditions are right. Techniques, resources and tools can help to speed up this process and make it more efficient but the development-over-time factor is also key.
* For the benefit of French speakers reading this post: make no bones about it = to state a fact in a way which allows no doubt.