First off, I want to address the decision over language - as I initially thought that you would want to use both languages on your website... but the more I researched, the more I realized that it is not as straightforward as, for example, debating whether or not to use English or Spanish for a website. The more I learned, the more I realized that answering this question requires one to be maniacally focused on their qualified customer, and how to give each visitor the best possible online experience, which can come in the form of information, offering relevant goods/services for sale - all while establishing trust and simultaneously taking steps to mitigate friction and anxiety, which may be unique for any underlying culture or group of peoples.
That all should sound familiar, right? The difference here is that the individual who asked the question (who, btw, is not stupid)) is on to something. In the United States, we typically target English queries without ever having to give it any further thought.
Let's step back and look at the history of the Sinhala language:
- the primary language of 70% of Sri Lankans, representing the largest ethnic group on the island
- For those who need numbers, this is approx: 70%+/- of the population, or roughly 13M people
- additionally, Sinhala is spoken as a second language among other ethnic groups on the island - therefore making it the most widely spoken language in Sri Lanka
This is where it get's interesting: Centuries of colonial rule has given rise to a melting pot or mixture of “loanwords” from several languages. These “loanwords” are borrowed from French, Portuguese, Dutch, and English Languages, to name a few. To make this even more complicated, Most of Sri Lankans are conversational in English. and begin learning English as the age that one would attend primary school.
In search of more data, I read through many travel sites, all which noted that it would be unlikely to face communication barriers should I visit. (BTW they are offering 5% off for Black Friday if anyone feels like booking a trip). Read more about Sinhala & Languages in Sri Lanka.
The individual who asked the question is facing a serious problem. Check out Google Trends for Sri Lanka & ask yourself how much sense this makes:

Look at those search terms. We are not dealing with a multilingual situation, nor is he targeting numerous countries. This is clearly the result of hundreds of years of outside influence, as mentioned earlier in this post.
This website, which is a dedicated directory of news sources and contains publications for each of their 3 main languages spoken (either independently, or mixed together, or .. you get the point.
Lanka Business Online said it best in a 2014 article:
Sri Lanka is home to over 20 million mobile phone subscribers and
close to 20 percent of the country’s population uses the internet.
With the embrace of technology more and more Sri Lankans are seen
engaging both domestic and international firms through e-business
channels in the pursuit of securing their day-to-day needs....
Local business can also be seen increasingly embracing online
e-business channels to cater to the growing base of digital consumers.
Sri Lankan mobile telecommunications service providers have been
seen introducing new contactless payment system to exploit the emerging
opportunities in the e-commerce sector.
By 2016, the number of internet users nearly doubled, reaching 29.3% of the population. Why all this extra information? To prove that the considerations of an online business are no different in the USA, nor Sri Lanka, regardless of how many languages are spoken.
- Research must be done, and we must strive to understand intent behind search queries
- Create a Google AdWords Account (regardless of you plan on advertising) and use Google's Free Keywords Tool to determine relative demand for the wide variety of keywords which may represent an individual who is searching for a product/service which your local business offers, or the "googler" is researching a problem that your local business can solve. Either way, the unfortuntate part about the long post is that you, my friend, must do your due diligence. I believe you when you say 90% of searches are in English. All good. But you still must write your content to appeal to your audience if you want to even have a chance at ranking (just don't use the wrong ccTLD).
If I were in your shoes, You must take a step back and identify exactly who your target market is. Are they a specific age group? Do they live in specific areas? Are they affluent? Middle class? Part of any affinity groups? Once you understand your customer, which more often than not is a process of trial & error + paying attention to your analytics, the data will uncover everything that you need to know to optimize your organic visibility and even improve your sales.
Before we go there, write the website for exactly who it's intended for. After all, you know best. And trust that Google is far ahead of the curve.
Wish you the best, please message me if you would like greater clarity about my suggestions/advice included above.