It's good news. Try this:
SELECT
CAST(b.start_time AS DATE) as date,
(a.source + '-' + a.report_item) as report_item,
SUM(CASE WHEN (b.lang = 'EN' AND b.sel_test = 1 AND a.report_item = 'Test')
OR (b.lang = 'EN' AND b.sel_test2 = 1 AND a.report_item = 'Test2')
THEN 1 ELSE 0 END) AS EN,
SUM(CASE WHEN (b.lang = 'BM' AND b.sel_test = 1 AND a.report_item = 'Test')
OR (b.lang = 'BM' AND b.sel_test2 = 1 AND a.report_item = 'Test2')
THEN 1 ELSE 0 END) AS BM
FROM ReportItem a
INNER JOIN TechDetails b
ON a.source = b.source
WHERE CAST(b.start_time AS DATE) >= @StartDate
AND CAST(b.start_time AS DATE) <= @EndDate
AND a.report_id = 8
AND a.flow_name = @flow_name
GROUP BY
CAST(b.start_time AS DATE),
a.flow_name,
a.report_item,
a.source
ORDER BY CAST(b.start_time AS DATE)
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden