More a rapid review than a systematic review!  Below is a list of articles that have mainly explored the effects of restricting the database used in a search.  At the bottom there are a smaller number of articles that have explored the impact of restricting the articles used to English-language.

Database restrictions

1. The comprehensiveness of Medline and Embase computer searches. Kleijnen J et al. Pharm Weekbl Sci. 1992 Oct 16;14(5):316-20.
2. Identifying randomized controlled trials of cognitive therapy for depression: Comparing the efficiency of Embase, Medline and PsycINFO bibliographic databases. Watson RJ et al. Br J Med Psychol. 1999 Dec;72 ( Pt 4):535-42
3. Identifying Clinical Trials in the Medical Literature with Electronic Databases: MEDLINE Alone Is Not Enough. Suarez-Almazor ME et al. Control Clin Trials. 2000 Oct;21(5):476-87.
4. Does the inclusion of grey literature influence estimates of intervention effectiveness reported in meta-analyses? McAuley L et al. Lancet 2000; 356: 1228–31
5. Lessons for search strategies from a systematic review, in The Cochrane Library, of nutritional supplementation trials in patients after hip fracture. Avenell A et al. Am J Clin Nutr. 2001 Mar;73(3):505-10.
6. Evaluation of the usefulness of Internet searches to identify unpublished clinical trials for systematic reviews. Eysenbach G et al. Med Inform Internet Med. 2001 Jul-Sep;26(3):203-18.
7. Comparison of bibliographic databases for information on the rehabilitation of people with severe mental illness. Brettle AJ et al. Bull Med Libr Assoc. 2001 Oct; 89(4): 353–362.
8. Evidence-based practice: extending the search to find material for the systematic review. Helmer D et al. Bull Med Libr Assoc. 2001 Oct; 89(4): 346–352.
9. Should meta-analysts search Embase in addition to Medline? Sampson M et al. J Clin Epidemiol. 2003 Oct;56(10):943-55.
10. Literature searching for randomized controlled trials used in Cochrane reviews: rapid versus exhaustive searches. Royle P and Milne R. Int J Technol Assess Health Care. 2003 Fall;19(4):591-603.
11. How important are comprehensive literature searches and the assessment of trial quality in systematic reviews? Egger M et al. Health Technol Assess. 2003;7(1):1-76.
12. Beyond Medline: reducing bias through extended systematic review search. Savoie I et al. Int J Technol Assess Health Care. 2003 Winter;19(1):168-78.
13. Searching multiple databases for systematic reviews: added value or diminishing returns? Stevinson C et al. Complement Ther Med. 2004 Dec;12(4):228-32.
14. A nursing qualitative systematic review required MEDLINE and CINAHL for study identification. Subirana M et al. J Clin Epidemiol. 2005 Jan;58(1):20-5.
15. Effectiveness of different databases in identifying studies for systematic reviews: experience from the WHO systematic review of maternal morbidity and mortality. Betrán AP et al. BMC Medical Research Methodology 2005, 5:6
16. Which resources should be used to identify RCT/CCTs for systematic reviews: a systematic review. Crumley ET et al. BMC Medical Research Methodology 2005, 5:24
17. A simplified search strategy for identifying randomised controlled trials for systematic reviews of health care interventions: a comparison with more exhaustive strategies. Royle P et al. BMC Med Res Methodol. 2005 Jul 23;5:23
18. Estimating the Horizon of Articles to Decide When to Stop Searching in Systematic Reviews: An Example Using a Systematic Review of RCTs Evaluating Osteoporosis Clinical Decision Support Tools. Kastner M et al. AMIA Annu Symp Proc. 2007; 2007: 389–393.
19. The capture–mark–recapture technique can be used as a stopping rule when searching in systematic reviews. Kastner M et al. J Clin Epidemiol. 2009 Feb;62(2):149-57.
20. How much searching is enough? Comprehensive versus optimal retrieval for technology assessments. Booth A. Int J Technol Assess Health Care. 2010 Oct;26(4):431-57
21. McMaster Premium LiteratUre Service (PLUS) performed well for identifying new studies for updated Cochrane reviews Hemens BJ, Haynes RB. J Clin Epidemiol. 2012 Jan;65(1):62-72.e1
22. A pragmatic strategy for the review of clinical evidence. Sagliocca L et al. J Eval Clin Pract. 2013 Aug;19(4):689-96.
23. Value of databases other than medline for rapid health technology assessments. International Journal of Technology Assessment in Health Care / Volume 30 / Issue 02 / April 2014
24. Pruning and prioritising: a case study of a pragmatic method for managing a rapid systematic review with limited resources. Harnan SE et al. Evidence & Policy: A Journal of Research, Debate and Practice. 30 December 2014
25. Using data sources beyond PubMed has a modest impact on the results of systematic reviews of therapeutic interventions. Halladay CW et al. J Clin Epidemiol. 2015 Sep;68(9):1076-84
26. The contribution of databases to the results of systematic reviews: a cross-sectional study. BMC Medical Research Methodology. 2016 16:127
27. Abbreviated literature searches were viable alternatives to comprehensive searches: a meta-epidemiological study. Nussbaumer-Streit B. et al. J Clin Epidemiol. 2018 Jun 1

English language restrictions

1. English-Language Restriction When Conducting Systematic Review-based Meta-analyses: Systematic Review of Published Studies. CADTH. 2009
2. The effect of English-language restriction on systematic review-based meta-analyses: a systematic review of empirical studies. Morrison A et al. Int J Technol Assess Health Care. 2012 Apr;28(2):138-44
3. Grey literature in systematic reviews: a cross-sectional study of the contribution of non-English reports, unpublished studies and dissertations to the results of meta-analyses in child-relevant reviews. Hartling L et al. BMC Medical Research Methodology 2017 17:64
4. Excluding non-English publications from evidence-syntheses did not change conclusions: a meta-epidemiological study. Nussbaumer-Streit B et al. 2019 Nov 4.